So, it seems it didn't come to my mind that builders might not have new enough rpm to use rpm.open() which is available since rpm 4.17.0. 

A fix for it ( https://src.fedoraproject.org/rpms/authselect/pull-request/15# ) has been merged, old authselect untagged, the new build of authselect should hopefully behave well now and builds of other packages should start working soon (after the local repo gets updated).