Replace deprecated tostring() call with tobytes()

[waveform80]

On booting a fresh hirsute daily image on a Raspberry Pi, the following appears in the MOTD:

=> There were exceptions while processing one or more plugins. See
/var/log/landscape/sysinfo.log for more information.

The log mentioned contains the following:

2021-02-19 15:27:28,062 ERROR    Network plugin raised an exception.
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/landscape/sysinfo/sysinfo.py", line 99, in run
    result = plugin.run()
  File "/usr/lib/python3/dist-packages/landscape/sysinfo/network.py", line 36, in run
    device_info = self._get_device_info()
  File "/usr/lib/python3/dist-packages/landscape/lib/network.py", line 163, in get_active_device_info
    speed, duplex = get_network_interface_speed(
  File "/usr/lib/python3/dist-packages/landscape/lib/network.py", line 249, in get_network_interface_speed
    res = status_cmd.tostring()
AttributeError: 'array.array' object has no attribute 'tostring'

Simply replacing the deprecated tostring() call with tobytes() fixes the issue and the MOTD message disappears.

LP: #1911050

[simpoir]

+1 LGTM