Add label parameter to pointplot

[mwaskom]

Unbreaks FacetGrid + pointplot (fixes #3004) and is generally useful.

g = sns.FacetGrid(tips, col="sex", hue="time")
g.map(sns.pointplot, "day", "total_bill")
g.add_legend()

Nevertheless, using pointplot and FacetGrid directly is still discouraged, prefer catplot.

[codecov]

Codecov Report

Merging #3016 (dc4829c) into master (54cab15) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3016   +/-   ##
=======================================
  Coverage   98.38%   98.38%           
=======================================
  Files          74       74           
  Lines       23648    23657    +9     
=======================================
+ Hits        23267    23276    +9     
  Misses        381      381           

Impacted Files	Coverage Δ
seaborn/categorical.py	98.54% <100.00%> (+<0.01%)	⬆️
tests/test_categorical.py	99.77% <100.00%> (+<0.01%)	⬆️