Avoid matplotlib 3.7.0 warnings

[MaozGelbart]

matplotlib==3.7.0 warns upon accessing the legend.legendHandles attribute, which is an undocumented legend attribute (see matplotlib/matplotlib#24538). This PR changes the usage of this attribute to the new documented way of accessing the handles (legend.legend_handles).

The fix involves a dedicated function in seaborn._compat module.

[codecov]

Codecov Report

Merging #3278 (780392f) into master (8735494) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3278   +/-   ##
=======================================
  Coverage   98.38%   98.38%           
=======================================
  Files          77       77           
  Lines       24705    24712    +7     
=======================================
+ Hits        24306    24313    +7     
  Misses        399      399           

Impacted Files	Coverage Δ
seaborn/_compat.py	50.00% <100.00%> (+1.96%)	⬆️
seaborn/axisgrid.py	97.20% <100.00%> (ø)
seaborn/utils.py	93.83% <100.00%> (+0.01%)	⬆️
tests/test_axisgrid.py	99.38% <100.00%> (+<0.01%)	⬆️
tests/test_categorical.py	99.74% <100.00%> (ø)
tests/test_relational.py	99.31% <100.00%> (ø)
tests/test_utils.py	98.86% <100.00%> (+<0.01%)	⬆️

[mwaskom]

Super helpful, thanks!