Update CI infrastructure

[astrofrog]

This switches to use GitHub Actions instead of Azure Pipelines and fixes a test failure with recent versions of Matplotlib (related to Figure.set_dpi no longer working).

I have also re-generated the baseline images for the tests with Matplotlib 3.8

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 🎉

Comparison is base (2a5551f) 88.41% compared to head (5d8381b) 88.43%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
+ Coverage   88.41%   88.43%   +0.02%     
==========================================
  Files          11       11              
  Lines         535      536       +1     
==========================================
+ Hits          473      474       +1     
  Misses         62       62              

Impacted Files	Coverage Δ
mpl_scatter_density/tests/__init__.py	100.00% <ø> (ø)
...atter_density/tests/test_scatter_density_artist.py	80.76% <100.00%> (+0.09%)	⬆️

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