Switch from appdirs to platformdirs

[domdfcoding]

Closes #10178

Changes of note:

tests/unit/test_appdirs.py

In appdirs, the platform was determined at import time but only checked when the functions were called. This allowed the global system attribute to be monkeypatched post-import to fake the current platform.

In platformdirs (post tox-dev/platformdirs#1) there are separate sets functions for each platform, and which ones get used is determined at import time. Therefore, although the system attribute exists, monkeypatching it no longer has the desired effect.

To work around this I have created three pytest fixtures (one each for Windows, macOS and Linux) which monkeypatch sys.platform and then reload both pip._vendor.platformdirs and pip._internal.utils.appdirs, so the import logic will run again. After the tests, the modules are re-reloaded to reset the state. If anyone can think of a better way I'm happy to hear it.

pkg_resources

Now patched as part of the vendoring process to use platformdirs.

macOS site_config_dir

Appdirs used to use /Library/Application Support/<app>, but it switched to /Library/Preferences/<app> in ActiveState/appdirs#100

Pip will now look in both locations for configuration

[domdfcoding]

Outstanding Issues

1. On Linux etc., pip patches appdirs to ignore an empty value for $XDG_CONFIG_DIRS and fallback to /etc/xdg/
   Per https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html this looks to be the right way of doing it, so I have added a patch for platformdirs.

   See also Linux: Empty environment variables should use defaults ActiveState/appdirs#130

2. Corrected *_config_dir() for OSX ActiveState/appdirs#100 changed appdirs.site_config_dir on macOS to use /Library/Preferences/ rather than /Library/Application Support/. There is a single test which checks the returned path and would need changing, but there's also the need to provide a migration path for users using the old path.

   Shall I patch platformdirs to maintain the current behaviour for now and come back to this later?

[uranusjr]

For the site_config_dir issue, IMO we should do something similar to Linux and add both /Library/Application Support/ and /Library/Preferences/. The order is probably not significant (we can always fix it if someone complains), just pick the one that’s easier to implement.

[pradyunsg]

@domdfcoding Would it be possible for you to update this PR?

Notably, looks like we don't need to patch platformdirs anymore.

[pradyunsg]

A gentle nudge.

[domdfcoding]

Sorry, working on this now.

[pradyunsg]

This PR needs a bunch of news/{name}.vendor.rst files, using all the package names, for the vendored packages which have been updated or otherwise modified.