Enable CI on Python 3.10

[97littleleaf11]

Description

Closes #11294

Test Plan

Check the tests still failed.

[pranavrajpal]

I think it might be better to just use virtualenv 20 always instead of just on python 3.10. #10855 seems to have added support for using virtualenv 20, so I think that PR just forgot to update the CI to use virtualenv 20.

[TH3CHARLie]

You may need to fix the build by fixing mypyc imports first

[97littleleaf11]

We could probably merge #11017 first

[JelleZijlstra]

We could probably merge #11017 first

Done!

[hauntsaninja]

Hooray!

[hauntsaninja]

Can we merge master for #11306

[pranavrajpal]

I think the usage of distutils that is causing a DeprecationWarning is in the setup.py mypyc generates to build C extensions:

mypy/scripts/mypyc

Lines 21 to 28 in 8cba3bc

	setup_format = """\
	from distutils.core import setup
	from mypyc.build import mypycify
	setup(name='mypyc_output',
	ext_modules=mypycify({}, opt_level="{}"),
	)
	"""

I'm not really sure what this file's for but I don't think this is what's causing the CI failure.

[97littleleaf11]

yeah, I'm looking at these. However I just wondering whether should I modify the test-data/packages/typedpkg-stubs/setup.py

[JelleZijlstra]

That's a sample package used for unit tests. We should probably update it too, but it should only affect a few unit tests.

[97littleleaf11]

Passes all tests now

[TH3CHARLie]

great job on making Python 3.10 working in CI