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_templates.py

@@ -6,6 +6,11 @@ from bs4 import BeautifulSoup
 from flask import url_for
 from freezegun import freeze_time
 from notifications_python_client.errors import HTTPError
+
+from app.main.views.templates import (
+    get_human_readable_delta,
+    get_last_use_message,
+)
 from tests import (
     single_notification_json,
     template_json,
@@ -22,11 +27,6 @@ from tests.conftest import (
 from tests.conftest import service_one as create_sample_service
 from tests.conftest import single_letter_contact_block
 
-from app.main.views.templates import (
-    get_human_readable_delta,
-    get_last_use_message,
-)
-
 
 @pytest.mark.parametrize('extra_args, expected_nav_links, expected_templates', [
     (