Fixed #19774 -- Deprecated the contenttypes.generic module.

It contained models, forms and admin objects causing undesirable
import side effects. Refs #16368.

Thanks to Ramiro, Carl and Loïc for the review.

tests/generic_relations/tests.py

@@ -1,7 +1,7 @@
 from __future__ import unicode_literals
 
 from django import forms
-from django.contrib.contenttypes.generic import generic_inlineformset_factory
+from django.contrib.contenttypes.forms import generic_inlineformset_factory
 from django.contrib.contenttypes.models import ContentType
 from django.test import TestCase
 from django.utils import six