DOC: clarify how to run tests in HACKING.txt
MAINT: altered mstats.sem to match stats.sem.  Closes gh-3086.

This adds a ``ddof`` keyword to the mstats version.  Output for
default ``ddof`` is unchanged.
Merge branch 'pr/3181' into master.

Reviewed at https://github.com/scipy/scipy/pull/3181