Set heatmap yticklabel alignment while rotating

[mwaskom]

Closes #2484

With the case from that issue:

m = ['January', 'February', 'March']
sns.heatmap(data=np.random.rand(3, 3), yticklabels=m, xticklabels=m)

Not sure of a good way to test this that isn't just asserting that the thing that's in the code happens, so I'm not adding a test. Should hopefully be able to catch it in the docs if it reappears.

[codecov]

Codecov Report

Merging #2574 (8aab749) into master (4f79753) will increase coverage by 0.18%.
The diff coverage is 100.00%.

❗ Current head 8aab749 differs from pull request most recent head ec6454a. Consider uploading reports for the commit ec6454a to get more accurate results

@@            Coverage Diff             @@
##           master    #2574      +/-   ##
==========================================
+ Coverage   97.27%   97.46%   +0.18%     
==========================================
  Files          17       17              
  Lines        6340     6341       +1     
==========================================
+ Hits         6167     6180      +13     
+ Misses        173      161      -12     

Impacted Files	Coverage Δ
seaborn/rcmod.py	100.00% <ø> (ø)
seaborn/matrix.py	95.87% <100.00%> (+<0.01%)	⬆️
seaborn/categorical.py	98.54% <0.00%> (+0.14%)	⬆️
seaborn/distributions.py	96.37% <0.00%> (+0.32%)	⬆️
seaborn/regression.py	97.79% <0.00%> (+0.62%)	⬆️
seaborn/_docstrings.py	100.00% <0.00%> (+12.50%)	⬆️

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