Cleaned up return of _get_cython_function

[WillAyd]

• closes CLN: Returned dtype of _get_cython_function unused #19557
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #19561 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #19561      +/-   ##
==========================================
+ Coverage    91.6%    91.6%   +<.01%     
==========================================
  Files         150      150              
  Lines       48750    48750              
==========================================
+ Hits        44656    44658       +2     
+ Misses       4094     4092       -2

Flag	Coverage Δ
#multiple	89.97% <100%> (ø)	⬆️
#single	41.75% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/groupby.py	92.17% <100%> (ø)	⬆️
pandas/util/testing.py	83.85% <0%> (+0.2%)	⬆️

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[jreback]

thanks!