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/test_common.py

@@ -5,7 +5,6 @@ from mongoengine import Document
 
 
 class TestCommon(unittest.TestCase):
-
     def test__import_class(self):
         doc_cls = _import_class("Document")
         self.assertIs(doc_cls, Document)