Replaces cElementTree with ElementTree

Ref: polybar/xpp#26

Replaces cElementTree with ElementTree

Ref: polybar/xpp#26

Breaking Changes:

* The new config parser imposes some restrictions on which characters can be
  used in section and key names. Users shouldn't be affected by this unless they
  use one of the following characters in any section or key name: `"'=;#[](){}:.$\%`
  Please consult [`man 5 polybar`](https://polybar.readthedocs.io/en/latest/man/polybar.5.html) for a full reference of the new config rules.
* `internal/temperature`: The first and last ramp element are now only used for
  `base-temperature` and below and `warn-temperature` and above respectively.
  This only slightly changes the ranges for which the different ramp levels are
  responsible for.
  (#2197)

Changelog

**Deprecations**

* `[settings]`: `throttle-input-for` has been removed. It wasn't a useful option
  and could cause certain actions (e.g. click commands) to be ignored. (#2117)

* All action names used by modules to handle click and scroll events are
  depercated (#1907).  This change mainly affects users of the menu module.
  Please read the [documentation](https://polybar.readthedocs.io/en/latest/user/actions.html) for instructions on how to migrate.

**New Config Options**

The `include-directory` key can be used the same as `include-file` and includes
all regular files in the given directory.

In labels:

* `label-NAME-minlen`, `label-NAME-alignment` can be used to pad labels with
  spaces to the right (alignment set to `left`), left (alignment set to
  `right`), and both sides (alignment set to `center`).

In `internal/backlight`:

* `enable-scroll` enables changing the brightness through scrolling.

In `internal/github`:

* `format-offline` is used when the module cannot connect to the server.
* `label-offline` can be used in `format-offline`.
* `api-url` can be used to connect to a custom github enterprise instance

In `internal/pulseaudio`:

* `click-right` and `click-middle` can be used to execute commands when
  right/middle clicking.
* `%decibels%` token can be used in `label-volume` and `label-muted` to show the
  volume in decibels.

**Changes To The Build System**

* Allow users to specify python executable when building. (polybar/xpp#27,
  #2125)
* The i3ipcpp submodule no longer rebuilds jsoncpp and just uses whatever
  version is available. (#2015, polybar/i3ipcpp#9)

**Features**

* New commandline argument: `-M` / `--list-all-monitors`.
    Will display all available monitors (even cloned ones).
* New log level: `notice`.
    Used as the default and is used for non-warning messages the user should
    nevertheless be aware of. (#2027)
* config:
    * New config parser (#1377)
    * `include-directory` key (#2196), see #1946
    * Add error message when an entire section is missing. (#2195)
    * `-minlen` and `-alignment` properties for labels. (#1546)
    * Make the `seperator` key in the bar section a label. (#1918)
    * Better color validation. (#1290)
* timer modules: Schedule module updates to be aligned with the update interval.
  For example, the date module now updates on the minute instead of in the
  middle of a minute if the interval is set to 60 seconds. (#2123), see #2064
* `custom/menu`: Multiple menu modules per bar (#1907)
* `internal/backlight`: Support for changing the brightness through scrolling.
  This may require additional changes to the system running polybar. (#1957)
* `internal/github`:
    * `format-offline` for when the module cannot connect to the server (#1825),
    see #1815
    * Support for github enterprise instances. (#1841), see #1812
* `internal/network`: Support `Gbit/s` for `%linkspeed%` token. (#2055)
* `internal/pulseaudio`:
    * `click-right` and `click-middle` keys (#1941)
    * `%decibels%` token (#1894), see #1886
* `internal/xworkspaces`: Proper implementation for `label-occupied`. (#822),
  see #874, #1444, #1033

**Fixes**

* Polybar not executing commands that produce output. (#1680), see #916
* Polybar froze until click commands finished executing (#2248)
* Polybar not properly working with mirrored monitors. (#1823), see #1192 and
  #1794
* Unstable animation framerate (#1683), see #568
* Multiple modules of the same type caused click events not to be delivered to
  the rigth one (#1907), see #1172
* config:
    * Seemingly unrelated error messages when BOM character appears in config.
    (#2166), see #2075
    * Fall back to next possible config location if config file doesn't exist
      (except if `--config` was used). (#2026), see #2016
* iconset: `fuzzy-match` chose first match, even if exact match was available.
  (#2042), see #2041
* `custom/menu`: Spacing issue (#1656)
* `internal/alsa`: Volume didn't go over 100% (#2184), see #2173
* `internal/backlight`: Use amdgpu workaround for all devices starting with
  `amdgpu_bl`. (#2122)
* `internal/battery`: Battery not marked as full if over `full-at` percent.
  (#2019), see #1622
* `internal/cpu`: More accurate cpu load calculation. (#1955)
* `internal/github`: Outdated GitHub API authentication. (#2029), see #2002
* `internal/memory`: Use the correct size prefixes (#2211), see #2023
* `internal/network`:
    * Wrong up- and downspeed values for non-integer intervals. (#2219)
    * tun/tap interfaces didn't query their IP addresses. (#1990), see #1986
    * Don't crash module if linkspeed cannot be queried. (#1772), see #1211
* `internal/temperature`:
    * `format-warn` was not used if the temperature was *exactly*
      `warn-temperature`. (#1897)
* `internal/xworkspaces`:
    * Multi-monitor issue (#1929), see #1764, #1849
    * Module sometimes showed too many workspaces (#1984), see #1983
* build:
    * xpp submodule doesn't work with python 3.9 (polybar/xpp#26)
    * CMake 3.17+ developer warnings (#2089, polybar/xpp#24, polybar/xpp#25)
    * gtest compilation failure (#1993), see google/googletest#2678
    * Compilation issue in GCC 6. (#1953)

Breaking Changes:

* The new config parser imposes some restrictions on which characters can be
  used in section and key names. Users shouldn't be affected by, this unless
  they use one of the following characters in any section or key name: `"'=;#[](){}:.$\%`
  Please consult [`man 5 polybar`](https://polybar.readthedocs.io/en/latest/man/polybar.5.html) for a full reference of the new config rules.
* `internal/temperature`: The first and last ramp element are now only used for
  `base-temperature` and below and `warn-temperature` and above respectively.
  This only slightly changes the ranges for which the different ramp levels are
  responsible for.
  (#2197)

Changelog

**Deprecations**

* `[settings]`: `throttle-input-for` has been removed. It wasn't a useful option
  and could cause certain actions (e.g. click commands) to be ignored. (#2117)

* All action names used by modules to handle click and scroll events are
  deprecated (#1907). This change mainly affects users of the menu module.
  Please read the [documentation](https://polybar.readthedocs.io/en/latest/user/actions.html) for instructions on how to migrate.

**New Config Options**

The `include-directory` key can be used the same as `include-file` and includes
all regular files in the given directory.

In labels:

* `label-NAME-minlen`, `label-NAME-alignment` can be used to pad labels with
  spaces to the right (alignment set to `left`), left (alignment set to
  `right`), and both sides (alignment set to `center`).

In `internal/backlight`:

* `enable-scroll` enables changing the brightness through scrolling.

In `internal/github`:

* `format-offline` is used when the module cannot connect to the server.
* `label-offline` can be used in `format-offline`.
* `api-url` can be used to connect to a custom github enterprise instance

In `internal/pulseaudio`:

* `click-right` and `click-middle` can be used to execute commands when
  right/middle clicking.
* `%decibels%` token can be used in `label-volume` and `label-muted` to show the
  volume in decibels.

**Changes To The Build System**

* Allow users to specify python executable when building. (polybar/xpp#27,
  #2125)
* The i3ipcpp submodule no longer rebuilds jsoncpp and just uses whatever
  version is available. (#2015, polybar/i3ipcpp#9)

**Features**

* New commandline argument: `-M` / `--list-all-monitors`.
    Will display all available monitors (even cloned ones).
* New log level: `notice`.
    Used as the default and is used for non-warning messages the user should
    nevertheless be aware of. (#2027)
* config:
    * New config parser (#1377)
    * `include-directory` key (#2196), see #1946
    * Add error message when an entire section is missing. (#2195)
    * `-minlen` and `-alignment` properties for labels. (#1546)
    * Make the `seperator` key in the bar section a label. (#1918)
    * Better color validation. (#1290)
* timer modules: Schedule module updates to be aligned with the update interval.
  For example, the date module now updates on the minute instead of in the
  middle of a minute if the interval is set to 60 seconds. (#2123), see #2064
* `custom/menu`: Multiple menu modules per bar (#1907)
* `internal/backlight`: Support for changing the brightness through scrolling.
  This may require additional changes to the system running polybar. (#1957)
* `internal/github`:
    * `format-offline` for when the module cannot connect to the server (#1825),
    see #1815
    * Support for github enterprise instances. (#1841), see #1812
* `internal/network`: Support `Gbit/s` for `%linkspeed%` token. (#2055)
* `internal/pulseaudio`:
    * `click-right` and `click-middle` keys (#1941)
    * `%decibels%` token (#1894), see #1886
* `internal/xworkspaces`: Proper implementation for `label-occupied`. (#822),
  see #874, #1444, #1033

**Fixes**

* Polybar not executing commands that produce output. (#1680), see #916
* Polybar froze until click commands finished executing (#2248)
* Polybar not properly working with mirrored monitors. (#1823), see #1192 and
  #1794
* Unstable animation framerate (#1683), see #568
* Multiple modules of the same type caused click events not to be delivered to
  the rigth one (#1907), see #1172
* config:
    * Seemingly unrelated error messages when BOM character appears in config.
    (#2166), see #2075
    * Fall back to next possible config location if config file doesn't exist
      (except if `--config` was used). (#2026), see #2016
* iconset: `fuzzy-match` chose first match, even if exact match was available.
  (#2042), see #2041
* `custom/menu`: Spacing issue (#1656)
* `internal/alsa`: Volume didn't go over 100% (#2184), see #2173
* `internal/backlight`: Use amdgpu workaround for all devices starting with
  `amdgpu_bl`. (#2122)
* `internal/battery`: Battery not marked as full if over `full-at` percent.
  (#2019), see #1622
* `internal/cpu`: More accurate cpu load calculation. (#1955)
* `internal/github`: Outdated GitHub API authentication. (#2029), see #2002
* `internal/memory`: Use the correct size prefixes (#2211), see #2023
* `internal/network`:
    * Wrong up- and downspeed values for non-integer intervals. (#2219)
    * tun/tap interfaces didn't query their IP addresses. (#1990), see #1986
    * Don't crash module if linkspeed cannot be queried. (#1772), see #1211
* `internal/temperature`:
    * `format-warn` was not used if the temperature was *exactly*
      `warn-temperature`. (#1897)
* `internal/xworkspaces`:
    * Multi-monitor issue (#1929), see #1764, #1849
    * Module sometimes showed too many workspaces (#1984), see #1983
* build:
    * xpp submodule doesn't work with python 3.9 (polybar/xpp#26)
    * CMake 3.17+ developer warnings (#2089, polybar/xpp#24, polybar/xpp#25)
    * gtest compilation failure (#1993), see google/googletest#2678
    * Compilation issue in GCC 6. (#1953)