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).

docs/code/tumblelog.py

@@ -1,16 +1,19 @@
 from mongoengine import *
 
-connect('tumblelog')
+connect("tumblelog")
+
 
 class Comment(EmbeddedDocument):
     content = StringField()
     name = StringField(max_length=120)
 
+
 class User(Document):
     email = StringField(required=True)
     first_name = StringField(max_length=50)
     last_name = StringField(max_length=50)
 
+
 class Post(Document):
     title = StringField(max_length=120, required=True)
     author = ReferenceField(User)
@@ -18,54 +21,57 @@ class Post(Document):
     comments = ListField(EmbeddedDocumentField(Comment))
 
     # bugfix
-    meta = {'allow_inheritance': True}
+    meta = {"allow_inheritance": True}
 
 
 class TextPost(Post):
     content = StringField()
 
+
 class ImagePost(Post):
     image_path = StringField()
 
+
 class LinkPost(Post):
     link_url = StringField()
 
+
 Post.drop_collection()
 
-john = User(email='jdoe@example.com', first_name='John', last_name='Doe')
+john = User(email="jdoe@example.com", first_name="John", last_name="Doe")
 john.save()
 
-post1 = TextPost(title='Fun with MongoEngine', author=john)
-post1.content = 'Took a look at MongoEngine today, looks pretty cool.'
-post1.tags = ['mongodb', 'mongoengine']
+post1 = TextPost(title="Fun with MongoEngine", author=john)
+post1.content = "Took a look at MongoEngine today, looks pretty cool."
+post1.tags = ["mongodb", "mongoengine"]
 post1.save()
 
-post2 = LinkPost(title='MongoEngine Documentation', author=john)
-post2.link_url = 'http://tractiondigital.com/labs/mongoengine/docs'
-post2.tags = ['mongoengine']
+post2 = LinkPost(title="MongoEngine Documentation", author=john)
+post2.link_url = "http://tractiondigital.com/labs/mongoengine/docs"
+post2.tags = ["mongoengine"]
 post2.save()
 
-print('ALL POSTS')
+print("ALL POSTS")
 print()
 for post in Post.objects:
     print(post.title)
-    #print '=' * post.title.count()
+    # print '=' * post.title.count()
     print("=" * 20)
 
     if isinstance(post, TextPost):
         print(post.content)
 
     if isinstance(post, LinkPost):
-        print('Link:', post.link_url)
+        print("Link:", post.link_url)
 
     print()
 print()
 
-print('POSTS TAGGED \'MONGODB\'')
+print("POSTS TAGGED 'MONGODB'")
 print()
-for post in Post.objects(tags='mongodb'):
+for post in Post.objects(tags="mongodb"):
     print(post.title)
 print()
 
-num_posts = Post.objects(tags='mongodb').count()
+num_posts = Post.objects(tags="mongodb").count()
 print('Found %d posts with tag "mongodb"' % num_posts)