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/organisations/test_organisation_invites.py

@@ -5,9 +5,9 @@ import pytest
 from bs4 import BeautifulSoup
 from flask import url_for
 from notifications_python_client.errors import HTTPError
-from tests.conftest import ORGANISATION_ID, normalize_spaces
 
 from app.notify_client.models import InvitedOrgUser
+from tests.conftest import ORGANISATION_ID, normalize_spaces
 
 
 def test_organisation_page_shows_all_organisations(