fix flake8.

Author: markusdap

graphql/validation/rules/fields_on_correct_type.py

@@ -3,7 +3,7 @@
 from ...error import GraphQLError
 from ...pyutils.ordereddict import OrderedDict
 from ...type.definition import (GraphQLInterfaceType, GraphQLObjectType,
-                                GraphQLUnionType, is_abstract_type)
+                                GraphQLUnionType)
 from ...utils.quoted_or_list import quoted_or_list
 from ...utils.suggestion_list import suggestion_list
 from .base import ValidationRule