requirements: Pin mypy and django-stubs version

Current djangos-stubs version (1.2.0) is not compatible with latest mypy one (0.750).
So pin their version until the issue is fixed.

requirements-test.txt

@@ -3,4 +3,4 @@ pytest-django
 swh.scheduler[testing]
 pytest-postgresql >= 2.1.0
 requests_mock
-django-stubs
+django-stubs < 1.3.0

tox.ini

@@ -31,6 +31,6 @@ setenv = DJANGO_SETTINGS_MODULE = swh.deposit.settings.testing
 extras =
   testing
 deps =
-  mypy
+  mypy < 0.750
 commands =
   mypy swh