[Merge] lp:~abompard/mailman/bug-1130696 into lp:mailman

[Aurélien Bompard]

Alright, I just changed that, and I tried generating a very simple subclass of MIMEMultipart and our Message instance. All tests seem to pass, but I'm not very familiar with the tricks of multiple inheritance / mixins.
I'm pushing the branch right now.
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https://code.launchpad.net/~abompard/mailman/bug-1130696/+merge/243226
Your team Mailman Coders is requested to review the proposed merge of lp:~abompard/mailman/bug-1130696 into lp:mailman.