Found solution in Using leapp to upgrade from Rocky 8.10 to Rocky 9 - #16 by dgoodfsa
Python 3.9 was used by Pulp in the past, but isn’t anymore. It should be safe to uninstall it via
dnf autoremoveordnf remove python39.
dnf autoremove did uninstall python39 with some other unneeded packages. Afterwards the upgrade did work.