Setup mailman3 server

We want to migrate to mailman3 as mailman2 is basically unmaintained and requires Python 2 which is EOL.

Because the mailman and mailman3 packages conflict and we don't want to perform a big bang migration, mailman3 must be deployed on a separate server. mailman-web (mailman3's web interface) hasn't been packaged yet, so for now we are using my homebrewed PKGBUILD.

[1] https://gist.github.com/klausenbusk/5982063f95c503754a51ed2fefb8915e

Ref #59 (closed)

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roles/mailman3/tasks/main.yml

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+# FIXME: python-zipp should be a dependency of mailman3: https://bugs.archlinux.org/task/71438
 - name: install mailman3, mailman3-hyperkitty, python-zipp, python-psycopg2, mailman-web and uwsgi-plugin-python
   pacman: name=mailman3,mailman3-hyperkitty,python-zipp,python-psycopg2,mailman-web,uwsgi-plugin-python state=present