According to https://pagure.io/koji/issue/971#comment-589393 there is one remaining yum dependence:
""" We do still have the mergerepos script that imports yum, but it is only run if:
merge_mode=simple is used for an external repo use_createrepo_c is set to False (default is now True)
"""
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/koji/koji/issues/1931
Please continue any further discussion there.