Add new bootstrapping bcond
    
    The Python RPM dependency generators now require python3-packaging and hence
    transitively python3-pyparsing.
    
    In order to bootstrap a new Python version, we need to build this without the
    generators first. The provides are added manually in that case.
    
    At that point, we cannot build the wheel or docs either,
    so this kinda swallowed the doc bcond.