Cleanup compat code for obsolete versions of dependencies

[hamdanal]

This PR removes compatibility code for features from matplotlib<1.4, numpy<1.20, and Python<3.8 as these versions are now obsolete according to the dependencies listed in pyrpoject.toml. There is one test change that was necessary, please see inline comment for explanation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (45a098f) 98.52% compared to head (dcb9b5d) 98.58%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3607      +/-   ##
==========================================
+ Coverage   98.52%   98.58%   +0.05%     
==========================================
  Files          75       75              
  Lines       24722    24697      -25     
==========================================
- Hits        24358    24348      -10     
+ Misses        364      349      -15     

Files	Coverage Δ
seaborn/_base.py	97.90% <100.00%> (+0.38%)	⬆️
seaborn/_compat.py	57.74% <ø> (-2.00%)	⬇️
seaborn/_core/plot.py	99.15% <100.00%> (-0.01%)	⬇️
seaborn/_core/properties.py	99.52% <100.00%> (+0.46%)	⬆️
seaborn/_statistics.py	99.69% <100.00%> (+<0.01%)	⬆️
seaborn/categorical.py	98.92% <100.00%> (+<0.01%)	⬆️
seaborn/distributions.py	96.42% <100.00%> (+<0.01%)	⬆️
seaborn/relational.py	99.65% <100.00%> (+<0.01%)	⬆️
seaborn/utils.py	96.26% <100.00%> (+2.08%)	⬆️
tests/_core/test_plot.py	98.84% <100.00%> (ø)
... and 2 more

[mwaskom]

Nice — thanks!