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_reference_field.py

@@ -26,15 +26,15 @@ class TestReferenceField(MongoDBTestCase):
         # with a document class name.
         self.assertRaises(ValidationError, ReferenceField, EmbeddedDocument)
 
-        user = User(name='Test User')
+        user = User(name="Test User")
 
         # Ensure that the referenced object must have been saved
-        post1 = BlogPost(content='Chips and gravy taste good.')
+        post1 = BlogPost(content="Chips and gravy taste good.")
         post1.author = user
         self.assertRaises(ValidationError, post1.save)
 
         # Check that an invalid object type cannot be used
-        post2 = BlogPost(content='Chips and chilli taste good.')
+        post2 = BlogPost(content="Chips and chilli taste good.")
         post1.author = post2
         self.assertRaises(ValidationError, post1.validate)
 
@@ -59,7 +59,7 @@ class TestReferenceField(MongoDBTestCase):
 
         class Person(Document):
             name = StringField()
-            parent = ReferenceField('self')
+            parent = ReferenceField("self")
 
         Person.drop_collection()
 
@@ -74,7 +74,7 @@ class TestReferenceField(MongoDBTestCase):
 
         class Person(Document):
             name = StringField()
-            parent = ReferenceField('self', dbref=True)
+            parent = ReferenceField("self", dbref=True)
 
         Person.drop_collection()
 
@@ -82,8 +82,8 @@ class TestReferenceField(MongoDBTestCase):
         Person(name="Ross", parent=p1).save()
 
         self.assertEqual(
-            Person._get_collection().find_one({'name': 'Ross'})['parent'],
-            DBRef('person', p1.pk)
+            Person._get_collection().find_one({"name": "Ross"})["parent"],
+            DBRef("person", p1.pk),
         )
 
         p = Person.objects.get(name="Ross")
@@ -97,21 +97,17 @@ class TestReferenceField(MongoDBTestCase):
 
         class Person(Document):
             name = StringField()
-            parent = ReferenceField('self', dbref=False)
+            parent = ReferenceField("self", dbref=False)
 
-        p = Person(
-            name='Steve',
-            parent=DBRef('person', 'abcdefghijklmnop')
+        p = Person(name="Steve", parent=DBRef("person", "abcdefghijklmnop"))
+        self.assertEqual(
+            p.to_mongo(), SON([("name", u"Steve"), ("parent", "abcdefghijklmnop")])
         )
-        self.assertEqual(p.to_mongo(), SON([
-            ('name', u'Steve'),
-            ('parent', 'abcdefghijklmnop')
-        ]))
 
     def test_objectid_reference_fields(self):
         class Person(Document):
             name = StringField()
-            parent = ReferenceField('self', dbref=False)
+            parent = ReferenceField("self", dbref=False)
 
         Person.drop_collection()
 
@@ -119,8 +115,8 @@ class TestReferenceField(MongoDBTestCase):
         Person(name="Ross", parent=p1).save()
 
         col = Person._get_collection()
-        data = col.find_one({'name': 'Ross'})
-        self.assertEqual(data['parent'], p1.pk)
+        data = col.find_one({"name": "Ross"})
+        self.assertEqual(data["parent"], p1.pk)
 
         p = Person.objects.get(name="Ross")
         self.assertEqual(p.parent, p1)
@@ -128,9 +124,10 @@ class TestReferenceField(MongoDBTestCase):
     def test_undefined_reference(self):
         """Ensure that ReferenceFields may reference undefined Documents.
         """
+
         class Product(Document):
             name = StringField()
-            company = ReferenceField('Company')
+            company = ReferenceField("Company")
 
         class Company(Document):
             name = StringField()
@@ -138,12 +135,12 @@ class TestReferenceField(MongoDBTestCase):
         Product.drop_collection()
         Company.drop_collection()
 
-        ten_gen = Company(name='10gen')
+        ten_gen = Company(name="10gen")
         ten_gen.save()
-        mongodb = Product(name='MongoDB', company=ten_gen)
+        mongodb = Product(name="MongoDB", company=ten_gen)
         mongodb.save()
 
-        me = Product(name='MongoEngine')
+        me = Product(name="MongoEngine")
         me.save()
 
         obj = Product.objects(company=ten_gen).first()
@@ -160,6 +157,7 @@ class TestReferenceField(MongoDBTestCase):
         """Ensure that ReferenceFields can be queried using objects and values
         of the type of the primary key of the referenced object.
         """
+
         class Member(Document):
             user_num = IntField(primary_key=True)
 
@@ -175,10 +173,10 @@ class TestReferenceField(MongoDBTestCase):
         m2 = Member(user_num=2)
         m2.save()
 
-        post1 = BlogPost(title='post 1', author=m1)
+        post1 = BlogPost(title="post 1", author=m1)
         post1.save()
 
-        post2 = BlogPost(title='post 2', author=m2)
+        post2 = BlogPost(title="post 2", author=m2)
         post2.save()
 
         post = BlogPost.objects(author=m1).first()
@@ -191,6 +189,7 @@ class TestReferenceField(MongoDBTestCase):
         """Ensure that ReferenceFields can be queried using objects and values
         of the type of the primary key of the referenced object.
         """
+
         class Member(Document):
             user_num = IntField(primary_key=True)
 
@@ -206,10 +205,10 @@ class TestReferenceField(MongoDBTestCase):
         m2 = Member(user_num=2)
         m2.save()
 
-        post1 = BlogPost(title='post 1', author=m1)
+        post1 = BlogPost(title="post 1", author=m1)
         post1.save()
 
-        post2 = BlogPost(title='post 2', author=m2)
+        post2 = BlogPost(title="post 2", author=m2)
         post2.save()
 
         post = BlogPost.objects(author=m1).first()