MNT: Bump Python and Sphinx, use Actions, new matrix

[pllim]

MNT: Bump supported Python and Sphinx versions.

TST: Update test matrix and use Actions.

• [x] Rerun/restart CI to get Release run too.
• [x] Fix failures.
• [x] Add codestyle job.

[codecov[bot]]

Codecov Report

Merging #139 (0efa1b4) into main (90b314e) will increase coverage by 89.00%. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           main     #139       +/-   ##
=========================================
+ Coverage      0   89.00%   +89.00%     
=========================================
  Files         0       22       +22     
  Lines         0     1091     +1091     
=========================================
+ Hits          0      971      +971     
- Misses        0      120      +120     

Impacted Files	Coverage Δ
sphinx_automodapi/automodsumm.py	85.67% <100.00%> (ø)
sphinx_automodapi/tests/helpers.py	100.00% <0.00%> (ø)
sphinx_automodapi/autodoc_enhancements.py	95.23% <0.00%> (ø)
sphinx_automodapi/tests/__init__.py	91.66% <0.00%> (ø)
...omodapi/tests/example_module/functions_with_ref.py	60.00% <0.00%> (ø)
sphinx_automodapi/smart_resolver.py	68.18% <0.00%> (ø)
sphinx_automodapi/automodapi.py	93.25% <0.00%> (ø)
...utomodapi/tests/example_module/abstract_classes.py	58.82% <0.00%> (ø)
sphinx_automodapi/tests/example_module/mixed.py	66.66% <0.00%> (ø)
... and 13 more

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[pllim]

Hmm, sphinx18 jobs for Linux and OSX are taking too long. I'll try to restart to make sure that is not transient.

[pllim]

Alas, the timeouts are persistent. Maybe I'll just bump Sphinx to 2+.