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/main/views/test_api_integration.py

@@ -5,6 +5,7 @@ from unittest.mock import call
 import pytest
 from bs4 import BeautifulSoup
 from flask import url_for
+
 from tests import validate_route_permission
 from tests.conftest import (
     SERVICE_ONE_ID,