Add more Mailman3 packages.

[ng0]

Hi,

I've commited some more Mailman3 related packages. I'm not sure,
but is Mailman3
a) python2 only?
b) python2 in the progress of migrating to python3?
c) mixed python2 and python3?

To me it seems like (c).
The commits are on the 'mail/mailman3' branch here:
https://pagure.io/guix-dev/commits/mail/mailman3

If you prefer them as direct patches via git format-patch && git
send, I can arrange that as well.
-- 
♥Ⓐ  ng0 -- https://www.inventati.org/patternsinthechaos/