Squash a large number of warnings in the tests

[mwaskom]

Various small edits to:

• Maintain forward compatibility with upcoming changes in dependencies
• Avoid triggering some numpy warnings around missing data and discrete mapping logic
• Silence a couple of innocuous warnings there's no clear way to work around

All that remains here are matplotlib/matplotlib#26300, matplotlib/matplotlib#26596 and pytest-dev/pytest-xdist#840 (decided to ignore the latter in the test config, since there's no apparent movement towards dealing with it upstream despite lots of reports).

[codecov]

Codecov Report

Merging #3447 (a03a90c) into master (b3e6b8d) will decrease coverage by 0.01%.
The diff coverage is 95.65%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3447      +/-   ##
==========================================
- Coverage   98.32%   98.32%   -0.01%     
==========================================
  Files          77       77              
  Lines       24413    24415       +2     
==========================================
+ Hits        24004    24005       +1     
- Misses        409      410       +1     

Files Changed	Coverage Δ
tests/test_distributions.py	99.67% <87.50%> (-0.07%)	⬇️
seaborn/_base.py	97.55% <100.00%> (-0.02%)	⬇️
seaborn/_core/plot.py	98.26% <100.00%> (ø)
seaborn/_core/properties.py	99.05% <100.00%> (ø)
seaborn/_stats/counting.py	100.00% <100.00%> (ø)
seaborn/categorical.py	98.90% <100.00%> (+<0.01%)	⬆️
seaborn/distributions.py	96.23% <100.00%> (-0.01%)	⬇️
tests/_stats/test_density.py	100.00% <100.00%> (ø)
tests/conftest.py	100.00% <100.00%> (ø)
tests/test_categorical.py	99.13% <100.00%> (ø)

[mwaskom]

Merging over codecov as it's just flagging a line that is version gated to a forthcoming matplotlib release.