configure: Allow (and prefer) Python 3
    
    Our build process no longer depends on Python 2, so we can
    finally allow Python 3 to satisfy our requirement for a Python
    interpreter.
    
    Since several distributions have now switched to installing
    Python 3 by default and Python 2 is on its way out, prefer the
    former when both are available.
    
    Signed-off-by: Andrea Bolognani <abologna@redhat.com>
    Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>