Migrate fully to pyproject.toml

[ml-evs]

This PR replaces setup.py and MANIFEST.in with a modern pyproject.toml file, as well as making many assorted tweaks to our CI and tests.

[codecov]

Codecov Report

Merging #1737 (f885928) into master (181dcd2) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1737      +/-   ##
==========================================
- Coverage   90.87%   90.85%   -0.03%     
==========================================
  Files          74       74              
  Lines        4615     4615              
==========================================
- Hits         4194     4193       -1     
- Misses        421      422       +1     

Flag	Coverage Δ
project	90.85% <ø> (-0.03%)	⬇️
validator	90.74% <ø> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
optimade/validator/__init__.py	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

[JPBergsma]

I did not see anything strange in this PR, although it is also not really my expertise.
The added comma in test_utils.py seems unnecessary, but is not important enough to block this PR.

[ml-evs]

The added comma in test_utils.py seems unnecessary, but is not important enough to block this PR.

Indeed, tell that to black! Thanks anyway