Mailman3 frontend number one: postorius.

[contact . ng0]

I've changed postorius to use python-2 only, but maybe I am wrong again.

Harmut have sent me this link and paste from it some days ago: 
<https://gitlab.com/mailman/mailman-bundler/issues/28#note_21121520>

Maybe only a fully functional Mailman-service will show wether this is wrong or 
right, but changing the used python version is trivial as it builds with both 
and I've added all 3rd level dependencies in both versions (py2 and py3) where 
available.