This commit converts the `configparser` package to use the
*pkgutil-style* namespace techniques.

It simply applies the changes from [the nixpkgs project][nix-fix], which
I found in [configparser issue 17][17].

There is more than one way to create a namespace in Python, [details
here][namespaces].

The important detail is that one can not mix *pkgutil-style* and
*pkg_resources-style* packages.

The `backports` package established the *backports* namespace as
`pkgutil-style` ([details here][backports]) and other backports
packages have followed its lead.

`configparser` using the *pkg_resource-style* has caused many
problems, see e.g. [configparser issue #17][17].

After applying the changes from the [nixpkgs project][nix-fix] I
updated the `tox.ini` file to use `py27,py36` (the only version I have
access to) and I believe that the tests passed (I'm not to `tox`).

```
(alice)[17:30:47]configparser>>tox
GLOB sdist-make: /Users/hartzell/tmp/configparser/setup.py
py27 create: /Users/hartzell/tmp/configparser/.tox/py27
py27 inst: /Users/hartzell/tmp/configparser/.tox/dist/configparser-3.6.0.zip
py27 installed: configparser==3.6.0
py27 runtests: PYTHONHASHSEED='2393890676'
py27 runtests: commands[0] | /Users/hartzell/tmp/configparser/.tox/py27/bin/python test_configparser.py
.................................................................................................................................................s.................................................................................................................s............................................../Users/hartzell/tmp/configparser/src/backports/configparser/__init__.py:988: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
  if key == self.default_section:
........
----------------------------------------------------------------------
Ran 314 tests in 0.460s

OK (skipped=2)
py27 runtests: commands[1] | /Users/hartzell/tmp/configparser/.tox/py27/bin/python test_helpers.py
py36 create: /Users/hartzell/tmp/configparser/.tox/py36
py36 inst: /Users/hartzell/tmp/configparser/.tox/dist/configparser-3.6.0.zip
py36 installed: configparser==3.6.0
py36 runtests: PYTHONHASHSEED='2393890676'
py36 runtests: commands[0] | /Users/hartzell/tmp/configparser/.tox/py36/bin/python test_configparser.py
.................................................................................................................................................s.................................................................................................................s...........................................ssss.......
----------------------------------------------------------------------
Ran 314 tests in 0.378s
  1 # example user config (see 'hg help config' for more info)

  1 Use pkgutil-style namespaces
OK (skipped=6)
py36 runtests: commands[1] | /Users/hartzell/tmp/configparser/.tox/py36/bin/python test_helpers.py
____________________________________________________ summary _____________________________________________________
  py27: commands succeeded
  1 Use pkgutil-style namespaces
  py36: commands succeeded
  congratulations :)
(alice)[17:31:05]configparser>>
```

[backports]: https://pypi.org/project/backports/
[namespaces]: https://packaging.python.org/guides/packaging-namespace-packages/
[nix-fix]: https://github.com/FRidh/nixpkgs/blob/master/pkgs/development/python-modules/configparser/0001-namespace-fix.patch
[17]: https://bitbucket.org/ambv/configparser/issues/17/importerror-when-used-with-other-backports

--HG--
branch : pkgutil-style