Add warn_singular parameter in kdeplot

[mwaskom]

Set to False to silence the warning about zero variance datasets.

One downside is that if you run into this in the histplot(..., kde=True) context the suggestion for how to disable it is incorrect (or at least unhelpful). Could potentially have histplot set this to False internally. In general I think I probably should have trusted my "non-deprecation-related warnings are almost always more annoying then helpful" intuition here.

Most cases where kdeplot is called internally now have this set to False, as I think the warning makes most sense in the "hey, why is my single plot empty" context, not when one kde curve in a large pairplot (or etc.) has disappeared.

Closes #2499

[codecov]

Codecov Report

Merging #2566 (3626481) into master (5ce48af) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2566   +/-   ##
=======================================
  Coverage   97.45%   97.45%           
=======================================
  Files          17       17           
  Lines        6326     6332    +6     
=======================================
+ Hits         6165     6171    +6     
  Misses        161      161           

Impacted Files	Coverage Δ
seaborn/axisgrid.py	95.63% <100.00%> (+0.01%)	⬆️
seaborn/distributions.py	96.37% <100.00%> (+<0.01%)	⬆️

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