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_activity.py

@@ -6,14 +6,14 @@ import pytest
 from bs4 import BeautifulSoup
 from flask import url_for
 from freezegun import freeze_time
+
+from app.main.views.jobs import get_status_filters, get_time_left
 from tests.conftest import (
     SERVICE_ONE_ID,
     mock_get_notifications,
     normalize_spaces,
 )
 
-from app.main.views.jobs import get_status_filters, get_time_left
-
 
 @pytest.mark.parametrize(
     "message_type,page_title", [