Rework our CI pipeline, to be purely GitHub Actions based

[pradyunsg]

Toward #7279
Closes #9087

AP hasn't been running on our recent PRs, Travis CI has "limited credits" that PyPA has had to ask for bumps to, and... GitHub Actions seems to just-work. We've already moved our MacOS workers and we have 20 workers for Linux+Windows; so might as well utilise them. :)

[pradyunsg]

Differences from status quo:

• Our Windows + Linux tests are back -- Azure Pipelines hasn't been running, for some reason.
• Tests are currently broken on PyPy3: see Migrate locations.py from distutils to sysconfig #9617 and https://github.com/pradyunsg/pip/runs/2250246469?check_suite_focus=true
• Moar conditional logic, to only runs tests/vendoring checks when necessary! ^>^
• Fail Fast on tests -- if one job fails, all others get a trigger telling them to cancel.

[pradyunsg]

FWIW, I plan on merging this in ~12-36 hours; since currently our CI pipelines aren't in a great shape and this is an improvement over status quo across the board (other than losing PyPy3 and a few Windows jobs, which we can add back in a follow up).

[pradyunsg]

FWIW, I've updated the branch protection rules, such that this is the only PR that can be merged at the moment.

[pradyunsg]

OK, so... I don't have the patience to wait 4 more hours here. I'm gonna merge this now. :)

I'm feeling fairly productive this morning and this is a significantly big improvement to our CI setup that help a lot with that.

[uranusjr]

@pradyunsg None of the test runs are marked as required now. Is it intentional?

[pradyunsg]

Is it intentional?

Yes, for now.

That's necessary since the runs get skipped if there's no code changes -- and in that case, their name doesn't show up in the checks -- see https://github.com/pypa/pip/actions/runs/711427044 from #9718's latest commit's checks. I don't have a good solution to this issue yet, but we're usually careful about merging PRs anyway; so it's fine for now.