Use PickleSerializer to serialize test sessions.

Use PickleSerializer so that we can serialize datetimes in sessions
(this is necessary to set expired sessions in automated tests). See
https://docs.djangoproject.com/en/2.0/topics/http/sessions/#write-your-own-serializer.

Author: martey

tests/tests.py

@@ -11,6 +11,7 @@
 
 class CleanupTest(TestCase):
     @override_settings(SESSION_ENGINE="django.contrib.sessions.backends.file")
+    @override_settings(SESSION_SERIALIZER="django.contrib.sessions.serializers.PickleSerializer")   # noqa: E501
     def test_session_cleanup(self):
         """
         Tests that sessions are deleted by the task