This commit:
1. Formats all of our existing code using `black`.
2. Adds a note about using `black` to `CONTRIBUTING.rst`.
3. Runs `black --check` as part of CI (failing builds that aren't properly formatted).
The inheritance model has changed, we no longer need to store an array of
`types` with the model we can just use the classname in `_cls`.
See the upgrade docs for information on how to upgrade
MongoEngine/mongoengine#148
