DeprecationWarning from patsy

[fpavogt]

Describe the bug
The patsy package, used by statsmodels, throws the following :

DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is 
deprecated since Python 3.3, and in 3.9 it will stop working
    from collections import Mapping

A fix is already implemented in the patsy source code: pydata/patsy#140

So we just need to wait for a new release of patsy onto pypi. Not sure when this will happen. But when it does, we should make sure to update our setup.py right away.