Merge pull request #48 from zopefoundation/tseaver-47-python_msg_seta…

…ttr_raise_attributeerror

fix: raise AttributeError from Python 'Message.__setattribute__'

src/zope/i18nmessageid/message.py

@@ -76,7 +76,7 @@ def __setattr__(self, key, value):
         It cannot be changed once the message id is created.
         """
         if getattr(self, '_readonly', False):
-            raise TypeError('readonly attribute')
+            raise AttributeError('readonly attribute')
         else:
             return str.__setattr__(self, key, value)