Add isort

[jmsmkn]

This PR adds isort with the following changes:

• [x] default_language_version removed
• [x] pyupgrade args set correctly
• [x] isort added before black
• [x] flake8-import-order removed
• [x] known_first_party set correctly
• [x] application-import-names and import-order-style removed

[jmsmkn]

The push tests are not passing due to #109, will try and get to that.