This is a problem for building Fedora package: if s390x builder picks the task, it will fail and have to be resubmitted. That is a problem that needs to be fixed.
I have disabled the test compose at RPM build time for now. It behaves strangely when python 3 is used.
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/pungi/pungi/issues/804
Please continue any further discussion there.