CLN: clean up groupby / categorical

[jreback]

xref #21151

[jreback]

cc @WillAyd
cc @jschendel

[codecov]

Codecov Report

Merging #21753 into master will decrease coverage by <.01%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master   #21753      +/-   ##
==========================================
- Coverage   91.93%   91.93%   -0.01%     
==========================================
  Files         159      160       +1     
  Lines       49733    49737       +4     
==========================================
+ Hits        45721    45724       +3     
- Misses       4012     4013       +1

Flag	Coverage Δ
#multiple	90.3% <96.29%> (-0.01%)	⬇️
#single	41.99% <0%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/arrays/categorical.py	95.93% <ø> (-0.09%)	⬇️
pandas/core/groupby/groupby.py	92.65% <100%> (-0.02%)	⬇️
pandas/core/groupby/categorical.py	95.65% <95.65%> (ø)
pandas/core/indexes/category.py	97.01% <0%> (ø)	⬆️

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[jreback]

cc @TomAugspurger ok?