The solution in my case, if any one of you ever come across this issue, was the following:
I had supervisord version 2.1 installed (via pip). I needed at least version 3.0 (because the "directory" setting (used in the supervisord.conf file) was introduced in the version 3.0.
I had to
pip uninstall supervisor
Then I had to
pip install supervisor==3.0
After that, I just had to set the desired configuration values in /etc/supervisord.conf, and my artisan
was running queue:listen
without problems.