CI: Prepare for python3 -> python
    
    rpm-spec-builddeps is used for extracting build dependencies from spec
    file to avoid duplication. But it was mostly used by python2 and
    therefore we did not notice issues with python3 which has "print"
    as a function and not as a statement.
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>