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/fields/test_long_field.py

@@ -13,23 +13,26 @@ from tests.utils import MongoDBTestCase
 
 
 class TestLongField(MongoDBTestCase):
-
     def test_long_field_is_considered_as_int64(self):
         """
         Tests that long fields are stored as long in mongo, even if long
         value is small enough to be an int.
         """
+
         class TestLongFieldConsideredAsInt64(Document):
             some_long = LongField()
 
         doc = TestLongFieldConsideredAsInt64(some_long=42).save()
         db = get_db()
-        self.assertIsInstance(db.test_long_field_considered_as_int64.find()[0]['some_long'], Int64)
+        self.assertIsInstance(
+            db.test_long_field_considered_as_int64.find()[0]["some_long"], Int64
+        )
         self.assertIsInstance(doc.some_long, six.integer_types)
 
     def test_long_validation(self):
         """Ensure that invalid values cannot be assigned to long fields.
         """
+
         class TestDocument(Document):
             value = LongField(min_value=0, max_value=110)
 
@@ -41,7 +44,7 @@ class TestLongField(MongoDBTestCase):
         self.assertRaises(ValidationError, doc.validate)
         doc.value = 120
         self.assertRaises(ValidationError, doc.validate)
-        doc.value = 'ten'
+        doc.value = "ten"
         self.assertRaises(ValidationError, doc.validate)
 
     def test_long_ne_operator(self):