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/test_utils.py

@@ -5,7 +5,6 @@ from pathlib import Path
 
 import pytest
 from freezegun import freeze_time
-from tests.conftest import fake_uuid
 
 from app import format_datetime_relative
 from app.utils import (
@@ -18,6 +17,7 @@ from app.utils import (
     get_cdn_domain,
     get_letter_timings,
 )
+from tests.conftest import fake_uuid
 
 
 def _get_notifications_csv(