Linting fixes and CI config for multiple Python 3.9+

[ml-evs]

Previously we had fail-fast: true (the default) which means that the first failing Python version would cancel all other tests. This PR changes it so that we can at least see which versions are failing and why (c.f. #33).

Also:

• Removed the template docs workflow, which has never been working.
• Use Python 3.9 specifically in pre-commit CI
• Fix any outstanding linting issues
• Fix dummy test on version number
• Tweak CI config to cancel jobs on pushes to the same PR
• Added a few cases of from __future__ import annotations that were missing for py39 compat
• Add a test that checks that the top-level of the package can be imported successfully

[ml-evs]

@gpetretto this PR addresses #33 and shows that we indeed currently fail on Python 3.8 (due to qtoolkit... so I've dropped it) and 3.9 (due to some weird typing issue with pydantic + future annotations messing up -- see the test I added). With the other changes bundled here (enforcing Python 3.9 pre-commit, etc.), hopefully this means the CI will be useful once again at detecting these issues, ready to add some proper tests.

I'll try now to fix the 3.9 issue with pydantic, but if you already have some idea how, feel free...

[utf]

As a side note, I found that I couldn't include the future annotations in modules defining pydantic models. So in atomate2 we remove the future import in those modules.

[ml-evs]

As a side note, I found that I couldn't include the future annotations in modules defining pydantic models. So in atomate2 we remove the future import in those modules.

Indeed, the issue as I understand it is that from __future__ import annotations is effectively just turning all annotations into postponed versions (i.e., strings), but when pydantic actually tries to use them it runs into issues. I think to support 3.9 we will have to downgrade the annotations in the config (as evidenced by the last test run).

[ml-evs]

@gpetretto this is ready for you to look at I think, unfortunately in the process of fixing the CI there are a lot of whitespace/file ending fixes, so to quickly summarise the actual code changes:

• use py39 compatible type hints for pydantic 8853680
• add stacklevels to warnings (some flake8 plugin was complaining but it does make sense) 8d24905
• add from __future__ import annotations in a few additional places -- if these still break then the next testing overhaul PR should catch them
• add a test that checks that settings etc can be imported

[gpetretto]

looks good to me. Thanks!