This change makes lib.mailman.get_mailman_client recognize settings.MAILMAN_REST_API_VERSION. If this value isn't set, it will fall back to 3.1 to provide backwards compatibility.
To make get_mailman_client a bit easier to test, the settings can now be passed as an argument, with django.conf.settings as default value.
However, the default value is still hardcoded and could probably be moved some place else. Maybe apps.DjangoMailman3Config would be a good place for that?