[CLN/TST] Follow instructions to remove function

[jbrockmendel]

Add a few files to the strict-linting list

[jbrockmendel]

Whoops, evidently forgot to update the lint file. Will add these in the next pass.

[coveralls]

Coverage increased (+0.003%) to 83.861% when pulling 7eab99f on jbrockmendel:main_tools5 into 8675b89 on statsmodels:master.

[codecov-io]

Codecov Report

Merging #4998 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4998      +/-   ##
==========================================
- Coverage   81.33%   81.32%   -0.01%     
==========================================
  Files         582      582              
  Lines       90026    90018       -8     
  Branches    10093    10091       -2     
==========================================
- Hits        73224    73211      -13     
+ Misses      14507    14506       -1     
- Partials     2295     2301       +6

Impacted Files	Coverage Δ
statsmodels/tools/transform_model.py	78.57% <ø> (-0.74%)	⬇️
statsmodels/tools/grouputils.py	61.2% <ø> (+0.69%)	⬆️
statsmodels/stats/sandwich_covariance.py	75.27% <100%> (+0.54%)	⬆️
statsmodels/tools/tests/test_transform_model.py	100% <100%> (ø)	⬆️
statsmodels/regression/mixed_linear_model.py	88.97% <0%> (-0.8%)	⬇️

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[jbrockmendel]

Whoops, evidently forgot to update the lint file. Will add these in the next pass.

@josef-pkt if there are no objections, I'd rather add that to the lint file in a separate PR and avoid CI clog since this is green

[josef-pkt]

looks fine to me, merging