Housekeeping

[bbengfort]

This PR fixes #1113 by handling some housekeeping issues. We've changed our master branch to the main branch and created a release issue template (though I don't think it is used automatically by GitHub). We've also checked the new pip resolver and all of our dependencies are compatible. This PR also updates our CI to test Python 3.8 and 3.9 to ensure we're testing on the latest version of Python.

This PR also addresses some warnings that were generated by the tests and fixes #707

This PR also adds a .gitattributes to fix #1127

Other housekeeping items:

• Apparently Sphinx add_stylesheet is deprecated so updated to use add_css_file instead.
• The cross-validation doc images were not building, so fixed the code by adding shuffle=True (scikit-learn deprecation)
• Adds a Sphinx build to GitHub ACtions for ensuring that the docs build correctly

[codecov[bot]]

Codecov Report

Merging #1187 (b56f64b) into develop (9b26ee2) will decrease coverage by 0.04%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1187      +/-   ##
===========================================
- Coverage    90.44%   90.40%   -0.05%     
===========================================
  Files           90       90              
  Lines         5078     5077       -1     
===========================================
- Hits          4593     4590       -3     
- Misses         485      487       +2     

Impacted Files	Coverage Δ
yellowbrick/contrib/missing/bar.py	88.88% <100.00%> (ø)
yellowbrick/features/rankd.py	91.08% <100.00%> (ø)
yellowbrick/cluster/elbow.py	97.74% <0.00%> (-0.76%)	:arrow_down:
yellowbrick/classifier/prcurve.py	97.76% <0.00%> (-0.75%)	:arrow_down:
yellowbrick/datasets/download.py	87.09% <0.00%> (-0.41%)	:arrow_down:
yellowbrick/regressor/residuals.py	89.71% <0.00%> (ø)

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