TST: restore a test that uses slogdet now that we have dropped support for older numpy versions
Merge pull request #4311 from argriffing/use-numpy-slogdet

TST: restore a test that uses slogdet now that we have dropped support for older numpy versions
MAINT: some fixes for stats.wishart added in gh-4135.

Fix docstring formatting and use np.linalg.slogdet where indicated by TODOs.
TST: clean up stats._multivariate tests a bit.