python/machine: use subprocess.DEVNULL instead of open(os.path.devnull)

python/machine: use subprocess.DEVNULL instead of open(os.path.devnull)

One less file resource to manage, and it helps quiet some pylint >=
2.8.0 warnings about not using a with-context manager for the open call.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-id: 20210527211715.394144-3-jsnow@redhat.com
Message-id: 20210517184808.3562549-3-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>