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

setup.cfg

@@ -1,10 +1,11 @@
 [tool:pytest]
-norecursedirs = node_modules bower_components
+norecursedirs=node_modules bower_components
 xfail_strict=true
 
 [isort]
 line_length=80
 indent='    '
 multi_line_output=3
-known_third_party = notifications_utils, notifications_python_client
-include_trailing_comma = True
+known_third_party=notifications_utils,notifications_python_client
+known_first_party=app,tests
+include_trailing_comma=True