Fix metaclass conflict on Django <= 1.6

django.db.models.manager.Manager is an instance of a subclass of `django.utils.deprecation.RenameMethodsBase` during the deprecation cycle of `get_query_set` in favor of `get_queryset`.

See django/django@6983a1a540a6e6c3bd941fa15ddd8cb49f9ec74e for more details.