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_datetime_field.py

@@ -19,10 +19,11 @@ class TestDateTimeField(MongoDBTestCase):
         Ensure an exception is raised when trying to
         cast an empty string to datetime.
         """
+
         class MyDoc(Document):
             dt = DateTimeField()
 
-        md = MyDoc(dt='')
+        md = MyDoc(dt="")
         self.assertRaises(ValidationError, md.save)
 
     def test_datetime_from_whitespace_string(self):
@@ -30,16 +31,18 @@ class TestDateTimeField(MongoDBTestCase):
         Ensure an exception is raised when trying to
         cast a whitespace-only string to datetime.
         """
+
         class MyDoc(Document):
             dt = DateTimeField()
 
-        md = MyDoc(dt='   ')
+        md = MyDoc(dt="   ")
         self.assertRaises(ValidationError, md.save)
 
     def test_default_value_utcnow(self):
         """Ensure that default field values are used when creating
         a document.
         """
+
         class Person(Document):
             created = DateTimeField(default=dt.datetime.utcnow)
 
@@ -48,8 +51,10 @@ class TestDateTimeField(MongoDBTestCase):
         person.validate()
         person_created_t0 = person.created
         self.assertLess(person.created - utcnow, dt.timedelta(seconds=1))
-        self.assertEqual(person_created_t0, person.created)  # make sure it does not change
-        self.assertEqual(person._data['created'], person.created)
+        self.assertEqual(
+            person_created_t0, person.created
+        )  # make sure it does not change
+        self.assertEqual(person._data["created"], person.created)
 
     def test_handling_microseconds(self):
         """Tests showing pymongo datetime fields handling of microseconds.
@@ -58,6 +63,7 @@ class TestDateTimeField(MongoDBTestCase):
 
         See: http://api.mongodb.org/python/current/api/bson/son.html#dt
         """
+
         class LogEntry(Document):
             date = DateTimeField()
 
@@ -103,6 +109,7 @@ class TestDateTimeField(MongoDBTestCase):
 
     def test_regular_usage(self):
         """Tests for regular datetime fields"""
+
         class LogEntry(Document):
             date = DateTimeField()
 
@@ -114,12 +121,12 @@ class TestDateTimeField(MongoDBTestCase):
         log.validate()
         log.save()
 
-        for query in (d1, d1.isoformat(' ')):
+        for query in (d1, d1.isoformat(" ")):
             log1 = LogEntry.objects.get(date=query)
             self.assertEqual(log, log1)
 
         if dateutil:
-            log1 = LogEntry.objects.get(date=d1.isoformat('T'))
+            log1 = LogEntry.objects.get(date=d1.isoformat("T"))
             self.assertEqual(log, log1)
 
         # create additional 19 log entries for a total of 20
@@ -150,8 +157,7 @@ class TestDateTimeField(MongoDBTestCase):
         self.assertEqual(logs.count(), 10)
 
         logs = LogEntry.objects.filter(
-            date__lte=dt.datetime(1980, 1, 1),
-            date__gte=dt.datetime(1975, 1, 1),
+            date__lte=dt.datetime(1980, 1, 1), date__gte=dt.datetime(1975, 1, 1)
         )
         self.assertEqual(logs.count(), 5)
 
@@ -159,6 +165,7 @@ class TestDateTimeField(MongoDBTestCase):
         """Ensure that invalid values cannot be assigned to datetime
         fields.
         """
+
         class LogEntry(Document):
             time = DateTimeField()
 
@@ -169,32 +176,32 @@ class TestDateTimeField(MongoDBTestCase):
         log.time = dt.date.today()
         log.validate()
 
-        log.time = dt.datetime.now().isoformat(' ')
+        log.time = dt.datetime.now().isoformat(" ")
         log.validate()
 
-        log.time = '2019-05-16 21:42:57.897847'
+        log.time = "2019-05-16 21:42:57.897847"
         log.validate()
 
         if dateutil:
-            log.time = dt.datetime.now().isoformat('T')
+            log.time = dt.datetime.now().isoformat("T")
             log.validate()
 
         log.time = -1
         self.assertRaises(ValidationError, log.validate)
-        log.time = 'ABC'
+        log.time = "ABC"
         self.assertRaises(ValidationError, log.validate)
-        log.time = '2019-05-16 21:GARBAGE:12'
+        log.time = "2019-05-16 21:GARBAGE:12"
         self.assertRaises(ValidationError, log.validate)
-        log.time = '2019-05-16 21:42:57.GARBAGE'
+        log.time = "2019-05-16 21:42:57.GARBAGE"
         self.assertRaises(ValidationError, log.validate)
-        log.time = '2019-05-16 21:42:57.123.456'
+        log.time = "2019-05-16 21:42:57.123.456"
         self.assertRaises(ValidationError, log.validate)
 
     def test_parse_datetime_as_str(self):
         class DTDoc(Document):
             date = DateTimeField()
 
-        date_str = '2019-03-02 22:26:01'
+        date_str = "2019-03-02 22:26:01"
 
         # make sure that passing a parsable datetime works
         dtd = DTDoc()
@@ -206,7 +213,7 @@ class TestDateTimeField(MongoDBTestCase):
         self.assertIsInstance(dtd.date, dt.datetime)
         self.assertEqual(str(dtd.date), date_str)
 
-        dtd.date = 'January 1st, 9999999999'
+        dtd.date = "January 1st, 9999999999"
         self.assertRaises(ValidationError, dtd.validate)
 
 
@@ -217,7 +224,7 @@ class TestDateTimeTzAware(MongoDBTestCase):
         connection._connections = {}
         connection._dbs = {}
 
-        connect(db='mongoenginetest', tz_aware=True)
+        connect(db="mongoenginetest", tz_aware=True)
 
         class LogEntry(Document):
             time = DateTimeField()
@@ -228,4 +235,4 @@ class TestDateTimeTzAware(MongoDBTestCase):
 
         log = LogEntry.objects.first()
         log.time = dt.datetime(2013, 1, 1, 0, 0, 0)
-        self.assertEqual(['time'], log._changed_fields)
+        self.assertEqual(["time"], log._changed_fields)