Format the codebase using Black (#2109)

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).

tests/document/__init__.py

@@ -9,5 +9,5 @@ from .instance import *
 from .json_serialisation import *
 from .validation import *
 
-if __name__ == '__main__':
+if __name__ == "__main__":
     unittest.main()