define isort first party (app and tests)

we were seeing isort produce different outputs locally and in docker -
this was due to it having different opinions about whether the tests
module (ie all our unit tests) is a first party (local) or third party
(pip installed) import. It's a first party import, so by defining this
in the setup.cfg isort settings, we can force it to be consistent
between environments.

Note: I don't know why it was different in the first place though

tests/app/notify_client/test_notification_client.py

@@ -1,9 +1,9 @@
 import uuid
 
 import pytest
-from tests import notification_json, single_notification_json
 
 from app.notify_client.notification_api_client import NotificationApiClient
+from tests import notification_json, single_notification_json
 
 
 @pytest.mark.parametrize("arguments,expected_call", [