The only reason it would try to use that lock file is if you are still trying to use the fhs layout. Edit your mailman.cfg file to use the dev layout. Location of your mailman.cfg should be under /var/lib/mailman/var/etc/.
You should probably also try to specify the full path to your config file with the -C option.
# su - mailman $ cd /var/lib/mailman $ source env/bin/activate (env) $ mailman info -C /var/lib/mailman/var/etc/mailman.cfg