use module name and class name for method instead of the reference to them

xdist/workermanage.py

@@ -429,8 +429,8 @@ def unserialize_warning_message(data):
         try:
             message = cls(*data["message_args"])
         except TypeError:
-            message_text = "{mod}.{class}: {msg}".format(mod=mod,
-                                                         cls=cls,
+            message_text = "{mod}.{class}: {msg}".format(mod=data["message_module"],
+                                                         cls=data["message_class_name"],
                                                          msg=data["message_str"])
             message = Warning(message_text)
     else: