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_send.py

@@ -16,6 +16,14 @@ from notifications_utils.template import (
     LetterImageTemplate,
     LetterPreviewTemplate,
 )
+from xlrd.biffh import XLRDError
+from xlrd.xldate import (
+    XLDateAmbiguous,
+    XLDateError,
+    XLDateNegative,
+    XLDateTooLarge,
+)
+
 from tests import (
     validate_route_permission,
     validate_route_permission_with_client,
@@ -38,13 +46,6 @@ from tests.conftest import (
     no_sms_senders,
     normalize_spaces,
 )
-from xlrd.biffh import XLRDError
-from xlrd.xldate import (
-    XLDateAmbiguous,
-    XLDateError,
-    XLDateNegative,
-    XLDateTooLarge,
-)
 
 template_types = ['email', 'sms']