app_python3 is packaged on:
1. CentOS 7; 1. Fedora 28, 29, 30; 1. OpenSUSE all supported version.
RHEL cannot be build, because some deps is not present on OBS service. But you can build by self using this [instruction](https://github.com/kamailio/kamailio/blob/master/pkg/kamailio/rhel/README.md).
Kamailio for RHEL 8 will be packaged when RHEL 8 will be available on OBS service.