COMPAT: statsmodel 0.9.0

[tdpetrou]

xref #18341

In the currently released version of statsmodels module statsmodels/discrete/discrete_model.py does this:

from pandas.core.api import get_dummies

@TomAugspurger fixed this here

In #18264 we tried to remove the get_dummies import from core.api and put it in core.reshape.api. This caused statsmodels to fail. We kept get_dummies in core.api with a TODO to remove it whenever statsmodels makes their next release.

We only have to delete this line as the import was kept in core.reshape.api

[TomAugspurger]

0.9 is out now.