Allow scipy='coo' in sparse_ratings

[reppertj]

Return a scipy.sparse.coo_matrix from sparse_ratings via the argument scipy='coo' (Fixes #213).

[codecov]

Codecov Report

Merging #214 (cfd3e8a) into master (2a3c10f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #214   +/-   ##
=======================================
  Coverage   92.42%   92.42%           
=======================================
  Files          43       43           
  Lines        3985     3987    +2     
=======================================
+ Hits         3683     3685    +2     
  Misses        302      302           

Impacted Files	Coverage Δ
lenskit/matrix/__init__.py	95.79% <100.00%> (+0.03%)	⬆️

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[mdekstrand]

Code looks good to me. I'd like to understand why tests failed on Mac with OpenBLAS - triggering a re-build to see if it's ephemeral.

[mdekstrand]

Upon inspection of the test suite failure, I believe it is unrelated - something perhaps changed in an OpenBLAS update to cause a subtle failure on Mac. I am re-running the master tests to confirm.

If that is the case, we'll be good to merge this change even with those tests failing.