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_dashboard.py

@@ -8,6 +8,15 @@ import pytest
 from bs4 import BeautifulSoup
 from flask import url_for
 from freezegun import freeze_time
+
+from app.main.views.dashboard import (
+    aggregate_status_types,
+    format_monthly_stats_to_list,
+    format_template_stats_to_list,
+    get_dashboard_totals,
+    get_free_paid_breakdown_for_billable_units,
+    get_tuples_of_financial_years,
+)
 from tests import (
     validate_route_permission,
     validate_route_permission_with_client,
@@ -19,15 +28,6 @@ from tests.conftest import (
     normalize_spaces,
 )
 
-from app.main.views.dashboard import (
-    aggregate_status_types,
-    format_monthly_stats_to_list,
-    format_template_stats_to_list,
-    get_dashboard_totals,
-    get_free_paid_breakdown_for_billable_units,
-    get_tuples_of_financial_years,
-)
-
 stub_template_stats = [
     {
         'template_type': 'sms',