Switch from appdirs to platformdirs

[gaborbernat]

We've forked appdirs under platformdirs https://github.com/platformdirs/platformdirs, is better maintain so I propose pip to switch part of its vendoring 👍

[DiddiLeija]

So, do you want to vendor platformdirs instead of appdirs?

[gaborbernat]

Yes that's my proposal.

[pradyunsg]

None of the paths or APIs changed, right?

[gaborbernat]

Yeah, fully backward compatible API-wise. We fixed some bugs, so paths might alter depending on what you're using, see https://github.com/platformdirs/platformdirs/blob/main/tests/test_comp_with_appdirs.py#L31-L32.

[uranusjr]

Tentitively adding this to 21.2 since I think this can be done as the vendor upgrade process.

[uranusjr]

I took a look at the compatibility test. Pip does not use user_log_dir at all, and does not pass the version argument to site_config_dir. So I think we are covered? I'll work on a PR for this.

[uranusjr]

Bad news, appdirs is used by pkg_resources, so we need to patch it. Not a showstopper, but it does mean the conversion would not be trivial. So I'm removing this from the 21.2 milestone to give it more time.

[domdfcoding]

I have made a start on the switch, including patching pkg_resources (see domdfcoding@f6513eb).

I'm working on the tests now, but they're proving troublesome because the current monkeypatching approach doesn't work with platformdirs.

[uranusjr]

One way we could make it work is to make all appdirs/platformdirs calls go through the pip._internals.utils wrapper, so we can just monkeypatch that module instead.