Change categorical auto gray parameter to "auto", with warning

[mwaskom]

There's a longstanding quirk of the categorical plots where line/edge colors can be specified as "gray" which actually triggers an automatic computation of a gray value that complements the main colors in the plot. Since "gray" is a valid matplotlib color and it's not very obvious that this is happening, this PR migrates the behavior to "auto", with one release cycle of backwards compatibility and a FutureWarning.

sns.boxenplot(tips, x="total_bill", hue="time", linecolor="auto", palette="pastel", gap=.1)

[codecov]

Codecov Report

Merging #3460 (7ce4da2) into master (ff446ee) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3460   +/-   ##
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  Coverage   98.33%   98.33%           
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  Files          75       75           
  Lines       24453    24460    +7     
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+ Hits        24045    24052    +7     
  Misses        408      408           

Files Changed	Coverage Δ
seaborn/categorical.py	98.90% <100.00%> (+<0.01%)	⬆️
tests/test_categorical.py	99.13% <100.00%> (+<0.01%)	⬆️