Enforce consistency in imports as part of build

This copies the config we use in the admin app, with a few changes
as discussed in the PR [1]. We'll apply these to our other apps.

[1]: https://github.com/alphagov/notifications-api/pull/3175#issuecomment-795530323

setup.cfg

@@ -7,3 +7,13 @@ exclude = venv*,__pycache__,node_modules,cache,migrations,build,sample_cap_xml_d
 max-line-length = 120
 # W504 line break after binary operator
 extend_ignore=B306, W504
+
+
+[isort]
+line_length=80
+indent='    '
+multi_line_output=3
+known_third_party=notifications_utils,notifications_python_client
+known_first_party=app,tests
+include_trailing_comma=True
+use_parentheses=True