Merge pull request #22522 from edx/diana/py3-cleaned-data

Remove items from a different dictionary than the one we're iterating over.

lms/djangoapps/course_api/blocks/serializers.py

@@ -104,9 +104,11 @@ class BlockSerializer(serializers.Serializer):  # pylint: disable=abstract-metho
         if authorization_denial_reason and authorization_denial_message:
             data['authorization_denial_reason'] = authorization_denial_reason
             data['authorization_denial_message'] = authorization_denial_message
+            cleaned_data = data.copy()
             for field in data.keys():  # pylint: disable=consider-iterating-dictionary
                 if field not in FIELDS_ALLOWED_IN_AUTH_DENIED_CONTENT:
-                    del data[field]
+                    del cleaned_data[field]
+            data = cleaned_data
 
         return data