Return device file stat.

def stat(self, filename, timeout=None):
    """Return device file stat."""
    transport = StatFilesyncTransport(self.stream)
    transport.write_data('STAT', filename, timeout)
    stat_msg = transport.read_message(timeout)
    stat_msg.assert_command_is('STAT')
    return DeviceFileStat(filename, stat_msg.mode, stat_msg.size, stat_msg.time)