Python 3 support for supervisord is still experimental and as mentioned in documentation, you shouldn't use it in production.
Mozilla Foundation has developed a package called circus which works on Python 3.
It manages process with circusd and it also has circusctl, circus-top, circus-web(similar to supervisor).
You can use your supervisor conf file with circus with some changes. It also has a seperate section in docs for users coming from supervisor. You can read that for more details.
Update:
Latest master has python 3 support. You can install it with
pip install git+https://github.com/Supervisor/supervisor