Move static project metadata to pyproject.toml, move/fix flake8 config

[mwtoews]

This PR moves the project metadata from setup.cfg to pyproject.toml as described by PEP 621 and implemented with setuptools >= 61.

Note that this metadata was previously moved with #73, but setuptools did not support PEP 621 at the time.

Other notes:

• "wheel" does not need to be listed as a build-system dependency anymore, so remove it
• The metadata format allows multiple authors, so use that feature
• Add a few keywords
• Add URL for documentation hosted at https://deltares.github.io/xmipy/xmipy.html
• Dependencies are alpha-sorted
• A shim setup.py is re-added, since this is still often expected, e.g. pip install -e . still needs this
• The package name is also in setup.py to avoid a GitHub bug
• setup.cfg is no-longer needed, so move the content to .flake8 and fix the formatting required by recent versions
• Fix the long line in xmipy/xmiwrapper.py, as identified by flake8
• Use install-gfortran-action for cross-platform CI tests

[codecov]

Codecov Report

Merging #84 (044338f) into develop (9697622) will decrease coverage by 0.09%.
The diff coverage is n/a.

❗ Current head 044338f differs from pull request most recent head b64aff7. Consider uploading reports for the commit b64aff7 to get more accurate results

@@             Coverage Diff             @@
##           develop      #84      +/-   ##
===========================================
- Coverage    87.50%   87.41%   -0.09%     
===========================================
  Files            5        6       +1     
  Lines          456      461       +5     
===========================================
+ Hits           399      403       +4     
- Misses          57       58       +1     

Impacted Files	Coverage Δ
xmipy/xmiwrapper.py	87.73% <ø> (-0.28%)	⬇️
xmipy/__init__.py	100.00% <0.00%> (ø)

[Hofer-Julian]

That is much nicer.
Thanks @mwtoews!