Required for running osbuild modules.
In Fedora, so far, python3 was always pulled by some other build-root
dependency. We should add it explicitly since it's required by all our
runners.
For RHEL and CentOS, the requirement is platform-python.
For RHEL 8 and CentOS 8, we also need to explicitly add python36 and for
RHEL 9 and CentOS 9 python3 because it's used to run the stages in
osbuild [1].
[1] ea8261cad6/runners/org.osbuild.rhel82 (L61)
