The package is not ported to Python 3 yet. You can visit Pagure for links to the upstream project. Ask for Python 3 status, and coordinate work with the upstream to port.
If upstream has Python 3 support but it is not yet in Fedora, get in touch with the Fedora maintainer (also visible in Pagure).
Mailman 3 with Python 3 support is ready but should be packaged separately. More info on the status of the new package can be found in a Fedora bug.