Monkeypatch some more generics. (#1050)

* Add generic monkeypatching for `FileProxyMixin`.

This fixes `TypeError: 'type' object is not subscriptable` for
`django.core.files.File` and `django.core.files.base.ContentFile`.

* Add generic monkeypatching for `ForeignKey`.

This matches the change coming in Django 4.1.

See https://github.com/django/django/pull/15571

django_stubs_ext/tests/test_monkeypatching.py

@@ -82,6 +82,8 @@ def test_patched_extra_classes_generics(make_generic_classes: _MakeGenericClasse
         (3, 0),
         (3, 1),
         (3, 2),
+        (4, 0),
+        (4, 1),
     ],
 )
 def test_patched_version_specific(