Hardcode %{_bindir} into python-config script + remove unreachable error handling
    
    The `dirname $0` breaks the script when it is linked to a different location,
    for example the RHEL's /usr/libexec/platform-python-config.
    
    The error handling was never reachable, failed exec ends the script.
    
    See https://src.fedoraproject.org/rpms/python3.9/pull-request/38