Fix histplot binwidth edgecases

[mwaskom]

Fixes #2721
Fixes #2785

These actually ended up being slightly distinct issues, although they can be fixed in the same block of code.

[codecov]

Codecov Report

Merging #2813 (9441604) into master (60e58eb) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2813   +/-   ##
=======================================
  Coverage   97.96%   97.96%           
=======================================
  Files          67       67           
  Lines       21745    21756   +11     
=======================================
+ Hits        21302    21313   +11     
  Misses        443      443           

Impacted Files	Coverage Δ
seaborn/axisgrid.py	96.05% <ø> (ø)
seaborn/regression.py	98.15% <ø> (ø)
seaborn/_statistics.py	100.00% <100.00%> (ø)
seaborn/tests/test_statistics.py	100.00% <100.00%> (ø)

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