Inherit errorbar alpha from scatter points in regplot

[mwaskom]

Closes #2540 by adding a test. Closes #2538.

sns.regplot(
    tips, x="size", y="total_bill",
    x_estimator=np.mean, scatter_kws={"alpha": .5}
)

[codecov]

Codecov Report

Merging #2853 (1c84c9a) into master (a674a83) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2853   +/-   ##
=======================================
  Coverage   98.12%   98.12%           
=======================================
  Files          69       69           
  Lines       22221    22228    +7     
=======================================
+ Hits        21804    21811    +7     
  Misses        417      417           

Impacted Files	Coverage Δ
seaborn/regression.py	99.07% <100.00%> (+<0.01%)	⬆️
tests/test_regression.py	99.53% <100.00%> (+<0.01%)	⬆️