Detect and prefer platform Python
    
    A platform Python interpreter is a special variant of the interpreter,
    that is only used for system software. It's located at
    /usr/libexec/platform-python.
    
    Fixes: https://pagure.io/freeipa/issue/7680
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>