How do I configure postfix_to_mailman.py?
First of all, postfix_to_mailman.py is a third party module neither created nor distributed by the GNU Mailman project. It would probably be rarely if ever used but for the fact that it is distributed by Debian (and hence Ubuntu) as part of their Mailman package.
postfix_to_mailman.py is part of an alternative to aliases for delivering list mail from Postfix to Mailman. It only works well in an environment in which all Mailman list addresses are in a dedicated domain in which there are no non-list addresses as postfix_to_mailman.py will only deliver to Mailman list addresses and a few others such as 'postmaster', 'abuse' and 'root', all of which get resent to the Mailman admin.
postfix_to_mailman.py is an alternative to and is incompatible with Mailman delivery via aliases. If you are configuring both Mailman aliases and postfix_to_mailman.py, you are making a mistake.
In the spirit of helping everyone who wants to use Mailman be successful in its use, those who support GNU Mailman will try to help you with your use of postfix_to_mailman.py, but please be aware of the following. Most issues with postfix_to_mailman.py are Postfix configuration issues, and the GNU Mailman project is not a primary resource for Postfix support. Most people are only aware of postfix_to_mailman.py because it comes with the Debian/Ubuntu Mailman package and the GNU Mailman project is not the primary resource for supporting Debian/Ubuntu packages. See this FAQ. The official position of the GNU Mailman project is that postfix_to_mailman.py offers no advantage over the officially supported Postfix-Mailman Integration.
Before trying to use postfix_to_mailman.py, you should see some of the hits at this Google search including http://mail.python.org/pipermail/mailman-users/2012-September/074017.html.