Washington Odhiambo via Mailman-users writes
pip install git+https://gitlab.com/mailman/mailman@master pip install git+https://gitlab.com/mailman/mailmanclient@master pip install git+https://gitlab.com/mailman/mailman-hyperkitty@master pip install git+https://gitlab.com/mailman/hyperkitty@master pip install git+https://gitlab.com/mailman/django-mailman3@master pip install git+https://gitlab.com/mailman/postorius@master pip install git+https://gitlab.com/mailman/mailman-web@master
these commands, run in sequence, rather than a
pip install git+https://gitlab.com/mailman/mailman@master git+https://gitlab.com/mailman/mailmanclient@master git+https://gitlab.com/mailman/mailman-hyperkitty@master git+https://gitlab.com/mailman/hyperkitty@master git+https://gitlab.com/mailman/django-mailman3@master git+https://gitlab.com/mailman/postorius@master git+https://gitlab.com/mailman/mailman-web@master
as may be implied by the documentation, work without a hitch!
Thanks and cheers!
-- Written by Thomas Krichel http://openlib.org/home/krichel on his 22214th day.