rhel-systemd: add python3-zstandard

[lzaoral]

Related: https://issues.redhat.com/browse/RHEL-122724

cc: @lnykryn

[yselkowitz]

Is this going to just be a dependency of systemd, or expected to be used directly in another context?

[lzaoral]

Only as a systemd dependency, I'm not aware of any other planned uses. Is it redundant to add it here?

Because systemd-ukify is in AppStream, python3-zstandard is going to be there as well. The guide says that any package in BaseOS/AppStream should be added to this repo.

[yselkowitz]

If it's going to be a hard (build or runtime) dependency but doesn't have standalone value, then there is no need to list it here. It will be added to Content Resolver output automatically once a build with the dependency has been created.

Please note that this dependency is currently conditional on Fedora-only and hence is not in ELN; that will need to be modified too:

https://src.fedoraproject.org/rpms/systemd/blob/rawhide/f/systemd.spec#_535