This was written assuming that the reader is familiar with Python software installation and tools like git and virtualenv.

I've been asked to provide more information about some of the things that may be unfamiliar to some users.

In the Migration section, I refer to a /opt/mailman/git/ directory containing clones of the various GitLab projects and the 'pull' and 'build' scripts to pull and install updates. First, you need git which can be installed via your OS package manager if you don't have it. The cloned projects themselves are created in the /opt/mailman/git/ directory. This is done using one of two git commands for each project. If you have a gitlab account with your SSH key(s), you can

cd /opt/mailman/git/
git clone git@gitlab.com:mailman/project_name.git

for each project. Otherwise, you can

cd /opt/mailman/git/
git clone https://gitlab.com/mailman/project_name.git

I also mention in the Installation section, creating a Python 3.6 virtualenv. It could just as well be Python 3.5 or Python 3.7 and should be the latest of those available on your server.

Creating the /opt/mailman/mm/venv virtualenv is done by

cd /opt/mailman/mm
virtualenv venv

or if you want to specify a Python other than the default

cd /opt/mailman/mm
virtualenv -p /path/to/desired/python venv

This is then activated by

source /opt/mailman/mm/venv/bin/activate

When active in a particular shell, it can be deactivated with the command

deactivate

or by exiting that shell.

If you don't have virtualenv, it can be installed with pip or via your OS package manager. For the latter, it might be a package named virtualenv or python3-virtualenv.