CI: Test on macOS arm64

[mayeut]

Fix #12908

The 2 failing tests mentioned in the issue are indeed present and marked as fail in the PR. The tests required virtualenv >= 20.0 to pass on python 3.8 & 3.9 on arm64 thus the requirements have been updated.

Side note: it seems all tests are passing with virtualenv >= 20.0 on other platforms. The pin to virtualenv<20.0 might not be needed anymore (introduced in #11288, @uranusjr might have some valuable insight about this).

[notatallshaw]

Well done, I somehow figured out and then subsequently forgot about virtualenv (https://github.com/pypa/pip/pull/12617#discussion_r1579796823).

[uranusjr]

IIRC virtualenv has <20 mostly just I didn’t want to risk rocking the boat too much on older platforms. I didn’t actually check if the CI works on older Python versions with new virtualenv at all.

If you are inclined to do some reviews (mostly needing to check tests that use the user site are testing the right things), we can probably remove the pin. I think we’re good to just keep the pins though, it’s less work.