Python 3.9.6: Error calling urllib.parse

With Python 3.9.6, [this change](https://github.com/python/cpython/blob/db3ff76da19004f266b62e98a81bdfd322861436/Lib/urllib/parse.py#L462) in `urllib.parse` causes an error using `urllib.parse.urlparse(..., scheme=None)` like here: https://github.com/spack/spack/blob/59028aa0a5d4f106c33955a9779ea3ae19864dd9/lib/spack/spack/util/url.py#L167-L168

I don't think this is a Python bug since the [docs](https://docs.python.org/3/library/urllib.parse.html#urllib.parse.urlparse) state:

> [scheme] should be the same type (text or bytes) as urlstring

However, [typeshed](https://github.com/python/typeshed/blob/c6b78354e6b9f979bb5b83044060b67e9c55a0ae/stdlib/urllib/parse.pyi#L125-L126) does have it typed as `Optional[str]` so I'm not sure.


### Steps to reproduce the issue

```console
$ python3 --version
Python 3.9.6
$ spack install zlib
==> Installing zlib-1.2.11-74mwnxgn6nujehpyyalhwizwojwn5zga
==> Error: Failed to install zlib due to AttributeError: 'NoneType' object has no attribute 'replace'
==> Error: 'NoneType' object has no attribute 'replace'
$ python3 -c "from urllib.parse import urlparse; urlparse('', scheme=None)"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/local/Cellar/python@3.9/3.9.6/Frameworks/Python.framework/Versions/3.9/lib/python3.9/urllib/parse.py", line 393, in urlparse
    splitresult = urlsplit(url, scheme, allow_fragments)
  File "/usr/local/Cellar/python@3.9/3.9.6/Frameworks/Python.framework/Versions/3.9/lib/python3.9/urllib/parse.py", line 462, in urlsplit
    scheme = scheme.replace(b, "")
AttributeError: 'NoneType' object has no attribute 'replace'
```

### Error Message

<details>
  <summary>Stacktrace</summary>

```console
$ spack --debug --stacktrace install zlib
Cellar/spack/0.16.2/lib/spack/spack/cmd/__init__.py:122 ==> [2021-06-30-19:23:27.332263] Imported install from built-in commands
Cellar/spack/0.16.2/lib/spack/spack/config.py:905 ==> [2021-06-30-19:23:27.336600] Reading config file /usr/local/Cellar/spack/0.16.2/etc/spack/defaults/config.yaml
Cellar/spack/0.16.2/lib/spack/spack/cmd/__init__.py:122 ==> [2021-06-30-19:23:27.370003] Imported install from built-in commands
Cellar/spack/0.16.2/lib/spack/spack/config.py:905 ==> [2021-06-30-19:23:27.379310] Reading config file /usr/local/Cellar/spack/0.16.2/etc/spack/defaults/repos.yaml
Cellar/spack/0.16.2/lib/spack/spack/config.py:905 ==> [2021-06-30-19:23:27.431903] Reading config file /usr/local/Cellar/spack/0.16.2/etc/spack/defaults/packages.yaml
Cellar/spack/0.16.2/lib/spack/spack/config.py:905 ==> [2021-06-30-19:23:27.463903] Reading config file /usr/local/Cellar/spack/0.16.2/etc/spack/defaults/darwin/packages.yaml
Cellar/spack/0.16.2/lib/spack/spack/config.py:905 ==> [2021-06-30-19:23:27.473058] Reading config file /Users/Branch/.spack/darwin/compilers.yaml
Cellar/spack/0.16.2/lib/spack/spack/database.py:361 ==> [2021-06-30-19:23:27.525082] DATABASE LOCK TIMEOUT: 3s
Cellar/spack/0.16.2/lib/spack/spack/database.py:365 ==> [2021-06-30-19:23:27.525475] PACKAGE LOCK TIMEOUT: No timeout
Cellar/spack/0.16.2/lib/spack/spack/installer.py:1371 ==> [2021-06-30-19:23:27.532093] Initializing the build queue from the build requests
Cellar/spack/0.16.2/lib/spack/spack/installer.py:989 ==> [2021-06-30-19:23:27.532547] Initializing the build queue for zlib
Cellar/spack/0.16.2/lib/spack/spack/installer.py:2054 ==> [2021-06-30-19:23:27.533780] Processing dependencies for zlib-1.2.11-74mwnxgn6nujehpyyalhwizwojwn5zga: ('build', 'link', 'run')
Cellar/spack/0.16.2/lib/spack/spack/database.py:461 ==> [2021-06-30-19:23:27.553180] Removing failure marking for zlib
Cellar/spack/0.16.2/lib/spack/spack/installer.py:1793 ==> [2021-06-30-19:23:27.553894] Pkg id zlib-1.2.11-74mwnxgn6nujehpyyalhwizwojwn5zga has the following dependents:
Cellar/spack/0.16.2/lib/spack/spack/installer.py:1377 ==> [2021-06-30-19:23:27.555060] Ensure all dependencies know all dependents across specs
Cellar/spack/0.16.2/lib/spack/spack/installer.py:935 ==> [2021-06-30-19:23:27.556576] Acquiring a write lock on zlib-1.2.11-74mwnxgn6nujehpyyalhwizwojwn5zga with timeout 1e-09
Cellar/spack/0.16.2/lib/spack/spack/stage.py:312 ==> [2021-06-30-19:23:27.561416] Creating stage lock spack-stage-zlib-1.2.11-74mwnxgn6nujehpyyalhwizwojwn5zga
Cellar/spack/0.16.2/lib/spack/spack/installer.py:1095 ==> [2021-06-30-19:23:27.563466] Installing zlib-1.2.11-74mwnxgn6nujehpyyalhwizwojwn5zga
Cellar/spack/0.16.2/lib/spack/spack/installer.py:393 ==> [2021-06-30-19:23:27.563803] Searching for binary cache of zlib-1.2.11-74mwnxgn6nujehpyyalhwizwojwn5zga
Cellar/spack/0.16.2/lib/spack/spack/config.py:905 ==> [2021-06-30-19:23:27.565923] Reading config file /usr/local/Cellar/spack/0.16.2/etc/spack/defaults/mirrors.yaml
Cellar/spack/0.16.2/lib/spack/spack/installer.py:1301 ==> [2021-06-30-19:23:27.569783] Flagging zlib-1.2.11-74mwnxgn6nujehpyyalhwizwojwn5zga as failed: 'NoneType' object has no attribute 'replace'
Cellar/spack/0.16.2/lib/spack/spack/installer.py:1577 ==> [2021-06-30-19:23:27.571197] Error: Failed to install zlib due to AttributeError: 'NoneType' object has no attribute 'replace'
Traceback (most recent call last):
  File "/usr/local/bin/spack", line 66, in <module>
    sys.exit(spack.main.main())
  File "/usr/local/Cellar/spack/0.16.2/lib/spack/spack/main.py", line 762, in main
    return _invoke_command(command, parser, args, unknown)
  File "/usr/local/Cellar/spack/0.16.2/lib/spack/spack/main.py", line 490, in _invoke_command
    return_val = command(parser, args)
  File "/usr/local/Cellar/spack/0.16.2/lib/spack/spack/cmd/install.py", line 365, in install
    install_specs(args, kwargs, zip(abstract_specs, specs))
  File "/usr/local/Cellar/spack/0.16.2/lib/spack/spack/cmd/install.py", line 207, in install_specs
    builder.install()
  File "/usr/local/Cellar/spack/0.16.2/lib/spack/spack/installer.py", line 1535, in install
    self._install_task(task)
  File "/usr/local/Cellar/spack/0.16.2/lib/spack/spack/installer.py", line 1101, in _install_task
    _install_from_cache(pkg, cache_only, explicit, unsigned,
  File "/usr/local/Cellar/spack/0.16.2/lib/spack/spack/installer.py", line 273, in _install_from_cache
    installed_from_cache = _try_install_from_binary_cache(
  File "/usr/local/Cellar/spack/0.16.2/lib/spack/spack/installer.py", line 394, in _try_install_from_binary_cache
    matches = binary_distribution.get_mirrors_for_spec(
  File "/usr/local/Cellar/spack/0.16.2/lib/spack/spack/binary_distribution.py", line 1396, in get_mirrors_for_spec
    results = try_direct_fetch(spec,
  File "/usr/local/Cellar/spack/0.16.2/lib/spack/spack/binary_distribution.py", line 1336, in try_direct_fetch
    buildcache_fetch_url = url_util.join(
  File "/usr/local/Cellar/spack/0.16.2/lib/spack/spack/util/url.py", line 156, in join
    obj = urllib_parse.urlparse(
  File "/usr/local/Cellar/python@3.9/3.9.6/Frameworks/Python.framework/Versions/3.9/lib/python3.9/urllib/parse.py", line 393, in urlparse
    splitresult = urlsplit(url, scheme, allow_fragments)
  File "/usr/local/Cellar/python@3.9/3.9.6/Frameworks/Python.framework/Versions/3.9/lib/python3.9/urllib/parse.py", line 462, in urlsplit
    scheme = scheme.replace(b, "")
AttributeError: 'NoneType' object has no attribute 'replace'
```
</details>

### Information on your system

* **Spack:** 0.16.2
* **Python:** 3.9.6
* **Platform:** darwin-bigsur-skylake

### Additional information

- [x] I have run `spack debug report` and reported the version of Spack/Python/Platform
- [x] I have searched the issues of this repo and believe this is not a duplicate
- [x] I have run the failing commands in debug mode and reported the output



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Python 3.9.6: Error calling urllib.parse #24644

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	obj = urllib_parse.urlparse(
	paths[j], scheme=None, allow_fragments=False)

Python 3.9.6: Error calling urllib.parse #24644

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