Fix BaseModelForm not working with generic type parameter (#705)

Resolves: https://github.com/typeddjango/django-stubs/issues/703

django_stubs_ext/django_stubs_ext/patch.py

@@ -6,6 +6,7 @@ from django.contrib.admin.options import BaseModelAdmin
 from django.db.models.fields import Field
 from django.db.models.manager import BaseManager
 from django.db.models.query import QuerySet
+from django.forms.models import BaseModelForm
 from django.views.generic.detail import SingleObjectMixin
 from django.views.generic.edit import FormMixin
 from django.views.generic.list import MultipleObjectMixin
@@ -44,6 +45,7 @@ _need_generic: List[MPGeneric[Any]] = [
     MPGeneric(MultipleObjectMixin),
     MPGeneric(BaseModelAdmin),
     MPGeneric(Field),
+    MPGeneric(BaseModelForm),
     # These types do have native `__class_getitem__` method since django 3.1:
     MPGeneric(QuerySet, (3, 1)),
     MPGeneric(BaseManager, (3, 1)),