Speed up import mocking in API doc generation
Speed up import mocking in API doc generation

Something about the internal imports of `pandas` caused our way of using
MagicMock to be very slow; this fix causes mock libraries to be returned
only for simple import errors, whereas other types of errors within
`__import__` will just be ignored.

acrellin/mltsp,acrellin/mltsp,acrellin/mltsp,acrellin/mltsp,bnaul/mltsp,bnaul/mltsp,acrellin/mltsp,bnaul/mltsp,mltsp/mltsp,mltsp/mltsp,bnaul/mltsp,bnaul/mltsp,mltsp/mltsp,bnaul/mltsp,mltsp/mltsp,acrellin/mltsp,mltsp/mltsp,mltsp/mltsp