Prefer stdlib contextlib over contextlib2 when available

[jdufresne]

ExitStack and suppress are available from stdlib contextlib on all
supported Python versions.

contextlib2 remains for the nullcontext context manager, which isn't
available in Python 3.6.

[uranusjr]

Is nullcontext the only remaining usage of contextlib2? This is trivial to implement, and we can (should?) drop the dependency entirely if that’s the case.

[jdufresne]

When I run tox -e vendoring, the file src/pip/_vendor/contextlib2.LICENSE.txt comes back. Is there something else I need to change?

[jdufresne]

Thanks for the review! The latest revision removes contextlib2 entirely and adds the suggested compat shim. This is used in testing only, so it is placed in the tests/ directory.

[pradyunsg]

Is there something else I need to change?

Not as far as I can see.

Is that behaviour still happening for you (I'd be surprised, since CI seems to be happy; and we're running it on CI)?

[jdufresne]

Okay, it looks like my /tmp/vendoring-cache/ directory contained stale data. After removing it, the license no longer comes back. Thanks.

[pradyunsg]

Ah, interesting! I'll investigate this sometime next year -- would you be able to file an issue over in pradyunsg/vendoring for this?

[uranusjr]

This will need to be rebased to master to fix the test suite for setuptools 52.0.

[uranusjr]

Rebase needed!

[BrownTruck]

Hello!

I am an automated bot and I have noticed that this pull request is not currently able to be merged. If you are able to either merge the master branch into this pull request or rebase this pull request against master then it will be eligible for code review and hopefully merging!

[jdufresne]

rebased