Catch PerfectSeparationWarning during regress_out

[jeskowagner]

• [x] Closes #3260
• [x] Tests included or not required because: minor change to maintain compat with statsmodels>=14.0

Inspired by @mwaskom's fix for seaborn, which promotes the warning to an error and catches it (in this case with the same logic scanpy was using for prior versions of statsmodels).