Ubuntu 21.04 (Hirsute) MOTD network section is broken

[Dominick Han]

```
Welcome to Ubuntu 21.04 (GNU/Linux 5.11.0-16-generic x86_64)

 * Documentation: https://help.ubuntu.com
 * Management: https://landscape.canonical.com
 * Support: https://ubuntu.com/advantage

  System information as of Fri Apr 23 14:15:44 EDT 2021

  System load: 4.09 Swap usage: 0% Users logged in: 0
  Usage of /: 7.9% of 438.62GB Temperature: 73.0 C
  Memory usage: 2% Processes: 241

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

 * Pure upstream Kubernetes 1.21, smallest, simplest cluster ops!

     https://microk8s.io/

0 updates can be installed immediately.
0 of these updates are security updates.

Last login: Fri Apr 23 12:04:04 2021 from 192.168.39.125
```
In the `sysinfo.log` file:
```
2021-04-23 17:13:14,425 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'
```

Upon investigation, `tostring()` method in Python array was deprecated in 3.9 (The default Python version in 21.04?)
On 20.10:
```
Python 3.8.6 (default, Jan 27 2021, 15:42:20)
[GCC 10.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from array import array
>>> array('B').tostring()
b''
```
On 21.04:
```
Python 3.9.4 (default, Apr 4 2021, 19:38:44)
[GCC 10.2.1 20210401] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from array import array
>>> array('B').tostring()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'array.array' object has no attribute 'tostring'
```

The fix would simply be replacing `tostring()` with `tobytes()`
`tostring` was deprecated since Python 3.2, and was removed in 3.9
https://docs.python.org/3.8/library/array.html#array.array.tostring

PS: I have upgraded my installation from 20.10 instead of brand new installation of the 21.04 release ISO. Not sure if that changes anything, the source I find here still uses `tostring()` https://packages.ubuntu.com/source/hirsute/landscape-client

From apt-cache policy:
```
landscape-client:
  Installed: (none)
  Candidate: 19.12-0ubuntu9
  Version table:
     19.12-0ubuntu9 500
        500 http://ca.archive.ubuntu.com/ubuntu hirsute/main amd64 Packages
```

[Ubuntu Foundations Team Bug Bot (crichton)]

The attachment "network.patch" seems to be a patch. If it isn't, please remove the "patch" flag from the attachment, remove the "patch" tag, and if you are a member of the ~ubuntu-reviewers, unsubscribe the team.

[This is an automated message performed by a Launchpad user owned by ~brian-murray, for any issues please contact him.]

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Jon N (nistorj)]

Hi,

The latest file shows as follows:

```
    try:
        fcntl.ioctl(sock, SIOCETHTOOL, packed) # Status ioctl() call
        if _PY3:
            res = status_cmd.tobytes()
        else:
            res = status_cmd.tostring()
        speed, duplex = struct.unpack("12xHB28x", res)
```

However even with this, or replacing and forcing only tobytes() still produces the following for us:
2021-08-18 03:13:20,074 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 158, in get_active_device_info
    flags = get_flags(sock, interface.encode())
  File "/usr/lib/python3/dist-packages/landscape/lib/network.py", line 136, in get_flags
    data = fcntl.ioctl(
OSError: [Errno 19] No such device

We have some our ip link has various dummy interfaces and some vrf info in it. not sure if that would cause issues with this or not.