is_multipart(): Use isinstance() instead of type equality.
diff --git a/Lib/email/Message.py b/Lib/email/Message.py
index db4b9a2..49ab266 100644
--- a/Lib/email/Message.py
+++ b/Lib/email/Message.py
@@ -107,7 +107,7 @@
 
     def is_multipart(self):
         """Return True if the message consists of multiple parts."""
-        if type(self._payload) is ListType:
+        if isinstance(self._payload, ListType):
             return True
         return False