Update pandas syntax to avoid deprecated behavior

[mwaskom]

[codecov[bot]]

Codecov Report

Merging #3040 (423cd5c) into master (d203329) will increase coverage by 0.00%. The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3040   +/-   ##
=======================================
  Coverage   98.25%   98.25%           
=======================================
  Files          69       69           
  Lines       23284    23285    +1     
=======================================
+ Hits        22878    22879    +1     
  Misses        406      406           

Impacted Files	Coverage Δ
seaborn/categorical.py	98.54% <100.00%> (ø)
seaborn/distributions.py	96.34% <100.00%> (+<0.01%)	:arrow_up:
tests/_stats/test_histogram.py	100.00% <100.00%> (ø)
tests/test_categorical.py	99.77% <100.00%> (ø)