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_platform_admin.py

@@ -4,14 +4,14 @@ from unittest.mock import ANY
 import pytest
 from bs4 import BeautifulSoup
 from flask import url_for
-from tests import service_json
-from tests.conftest import mock_get_user, normalize_spaces
 
 from app.main.views.platform_admin import (
     create_global_stats,
     format_stats_by_service,
     sum_service_usage,
 )
+from tests import service_json
+from tests.conftest import mock_get_user, normalize_spaces
 
 
 @pytest.mark.parametrize('endpoint', [