MAINT: Remove string_types in favor of str

[bashtage]

Remove old alias

[codecov]

Codecov Report

Merging #5972 into master will increase coverage by <.01%.
The diff coverage is 80.82%.

@@            Coverage Diff             @@
##           master    #5972      +/-   ##
==========================================
+ Coverage   83.23%   83.23%   +<.01%     
==========================================
  Files         610      610              
  Lines       95699    95657      -42     
  Branches    10482    10483       +1     
==========================================
- Hits        79654    79624      -30     
+ Misses      13639    13627      -12     
  Partials     2406     2406

Impacted Files	Coverage Δ
statsmodels/graphics/plot_grids.py	0% <ø> (ø)	⬆️
...odels/sandbox/panel/sandwich_covariance_generic.py	0% <ø> (ø)	⬆️
statsmodels/genmod/families/tests/test_link.py	100% <ø> (ø)	⬆️
statsmodels/sandbox/tsa/example_arma.py	0% <ø> (ø)	⬆️
statsmodels/stats/contrast.py	80.44% <ø> (-0.09%)	⬇️
statsmodels/distributions/mixture_rvs.py	65.21% <ø> (-0.5%)	⬇️
statsmodels/tools/linalg.py	73.07% <ø> (-0.51%)	⬇️
statsmodels/stats/multitest.py	93.44% <ø> (-0.03%)	⬇️
statsmodels/tools/data.py	72.88% <ø> (-0.46%)	⬇️
statsmodels/stats/_adnorm.py	80.7% <ø> (-0.34%)	⬇️
... and 133 more

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[coveralls]

Coverage increased (+1.0e-05%) to 85.748% when pulling 7252192 on bashtage:string_types into 3e5e92d on statsmodels:master.

[jbrockmendel]

Looks like a complaint somewhere about asstr2