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/all_warnings/__init__.py

@@ -9,34 +9,32 @@ import warnings
 from mongoengine import *
 
 
-__all__ = ('AllWarnings', )
+__all__ = ("AllWarnings",)
 
 
 class AllWarnings(unittest.TestCase):
-
     def setUp(self):
-        connect(db='mongoenginetest')
+        connect(db="mongoenginetest")
         self.warning_list = []
         self.showwarning_default = warnings.showwarning
         warnings.showwarning = self.append_to_warning_list
 
     def append_to_warning_list(self, message, category, *args):
-        self.warning_list.append({"message": message,
-                                  "category": category})
+        self.warning_list.append({"message": message, "category": category})
 
     def tearDown(self):
         # restore default handling of warnings
         warnings.showwarning = self.showwarning_default
 
     def test_document_collection_syntax_warning(self):
-
         class NonAbstractBase(Document):
-            meta = {'allow_inheritance': True}
+            meta = {"allow_inheritance": True}
 
         class InheritedDocumentFailTest(NonAbstractBase):
-            meta = {'collection': 'fail'}
+            meta = {"collection": "fail"}
 
         warning = self.warning_list[0]
         self.assertEqual(SyntaxWarning, warning["category"])
-        self.assertEqual('non_abstract_base',
-                         InheritedDocumentFailTest._get_collection_name())
+        self.assertEqual(
+            "non_abstract_base", InheritedDocumentFailTest._get_collection_name()
+        )