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_notify_admin_api_client.py

@@ -3,10 +3,10 @@ from unittest.mock import patch
 
 import pytest
 import werkzeug
-from tests import service_json
-from tests.conftest import api_user_active, platform_admin_user, set_config
 
 from app.notify_client import NotifyAdminAPIClient
+from tests import service_json
+from tests.conftest import api_user_active, platform_admin_user, set_config
 
 SAMPLE_API_KEY = '{}-{}'.format('a' * 36, 's' * 36)