Move to mailman3

[Aurelien Bompard]

OK people, thanks for your enthusiasm, I'll migrate infra in the next hours :-)
I'll be online this weekend to fix things if they break, and there's
an easy way to rollback if it's unfixable.

The only distrubing thing that I've seen is mail delivery being
stopped sometimes when an ansible playbook is run on the servers (but
not always of course, that would be too easy). But no mail is lost, a
simple restart of mailman gets it rolling again.

I'll send an email in this thread when the migration is done.

Aurélien