debian/control: Bump minimum required django and pytest-django

As for the webapp, ensure django 2.2 from backports is used on buster
as everything is already setup in puppet to have it in production.

debian/control

@@ -7,12 +7,12 @@ Build-Depends: debhelper (>= 9),
                python3-all,
                python3-click,
                python3-swh.auth.django,
-               python3-django,
+               python3-django (>= 2:2.2.11~),
                python3-djangorestframework,
                python3-django-test-migrations,
                python3-iso8601,
                python3-pytest,
-               python3-pytest-django,
+               python3-pytest-django (>= 3.5.1~),
                python3-pytest-mock,
                python3-pytest-postgresql,
                python3-swh.core.db.pytestplugin,