Make UUID compatible (Fixed issue #936) (#937)

* Make UUID compatible

* Fix typo

* black code formatting

Author: ekampf

graphene/types/uuid.py

@@ -1,5 +1,5 @@
 from __future__ import absolute_import
-
+import six
 from uuid import UUID as _UUID
 
 from graphql.language import ast
@@ -12,8 +12,9 @@ class UUID(Scalar):
 
     @staticmethod
     def serialize(uuid):
-        if isinstance(uuid, str):
+        if isinstance(uuid, six.string_types):
             uuid = _UUID(uuid)
+
         assert isinstance(uuid, _UUID), "Expected UUID instance, received {}".format(
             uuid
         )