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).
It covers operations such as:
1. Document initialization.
2. Accessing/setting attributes on a Document instance.
3. Serializing a Document instance (via `to_mongo`).
4. Deserializing a Document instance (via `_from_son`).
5. Serializing + saving a Document in the database (via `save`).
5. Loading a Document from the database + deserializing (via `Doc.objects[0]`).
And it does so for both basic flat documents and more complex nested docs.
