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/accounts/test_choose_accounts.py

@@ -1,5 +1,6 @@
 import pytest
 from flask import url_for
+
 from tests.conftest import normalize_spaces
 
 SAMPLE_DATA = {

tests/app/main/views/accounts/test_show_accounts_or_dashboard.py

@@ -1,8 +1,8 @@
 import pytest
 from flask import url_for
-from tests import user_json
 
 from app.notify_client.models import User
+from tests import user_json
 
 
 def user_with_orgs_and_services(num_orgs, num_services, platform_admin=False):