Enable matplotlib legend loc values for embeddings

[ivirshup]

Fixes #2267

TODO

• [ ] Tests
• [ ] Changelog

[codecov[bot]]

Codecov Report

Merging #2277 (9dcf5e4) into master (394191f) will decrease coverage by 0.01%. The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master    #2277      +/-   ##
==========================================
- Coverage   71.82%   71.80%   -0.02%     
==========================================
  Files          98       98              
  Lines       11539    11543       +4     
==========================================
+ Hits         8288     8289       +1     
- Misses       3251     3254       +3     

Impacted Files	Coverage Δ
scanpy/plotting/_tools/scatterplots.py	87.64% <25.00%> (-0.61%)	:arrow_down:

[ktpolanski]

Ran into this today as a coworker wanted to stick a UMAP legend into lower left and couldn't. I whipped up a hotfix, which turned out to be exactly the same syntax as you've got going on here, and was on my way to mention it when I found this.

Would be nice to see this integrated as the docs imply this is possible, and it seems useful to have on occasion?