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_manage_users.py

@@ -3,6 +3,10 @@ import copy
 import pytest
 from bs4 import BeautifulSoup
 from flask import url_for
+
+import app
+from app.notify_client.models import InvitedUser
+from app.utils import is_gov_user
 from tests.conftest import (
     SERVICE_ONE_ID,
     active_user_manage_template_permission,
@@ -13,10 +17,6 @@ from tests.conftest import (
 )
 from tests.conftest import service_one as create_sample_service
 
-import app
-from app.notify_client.models import InvitedUser
-from app.utils import is_gov_user
-
 
 @pytest.mark.parametrize('user, expected_self_text, expected_coworker_text', [
     (