Synchronise tested Python versions in tox.ini and .github/workflows/tox.yml

[JustAnotherArchivist]

actions/toolkit#437 didn't use the same Python version in the GitHub Actions config as in the tox config. As a result, Python 2 and 3.5 aren't tested automatically on PRs etc.

This synchronises the two lists. Namely, it restores CPython 2 and 3.5 testing on GitHub Actions and adds PyPy to the tox configuration.

As expected (#435), installations on the versions 2.7 and 3.5 do not currently work due to dependencies no longer supporting those EOL versions, so those jobs will fail until they are fixed in further PRs. Unfortunately, GitHub Actions still doesn't support allowable failures (actions/runner#2347), so this will show red crosses everywhere. I think that's preferable to not running the tests at all though, otherwise the code will likely break even further under those ancient Python versions, which makes a final compatibility release as desired by @jjjake in actions/toolkit#435 harder.

NB, I made this commit on top of the v2.2.0 release, which isn't on master yet. actions/toolkit#449 should likely be merged before this PR.

[cclauss]

To make tox ✅ on the legacy Pythons, please edit tests/requirements.txt to make similar changes as pytest in that file. Something like:

flake8; python_version < '3.6'
flake8>=4.0.1; python_version >= '3.6'

[JustAnotherArchivist]

Yeah, I already fixed it on another branch, actually, but obviously based on this commit so the 2.7/3.5 tests run at all, so I figured I'd make a separate PR once this is merged to avoid confusion.

[JustAnotherArchivist]

Nevermind, it's cleaner to do both in one, so this is superseded by actions/toolkit#451.