Fix grgsm_scanner -l

[0xxstone]

When running the command grgsm_scanner -l, the following exception is raised:

Traceback (most recent call last):
  File "/usr/bin/grgsm_scanner", line 797, in <module>
    main()
  File "/usr/bin/grgsm_scanner", line 774, in main
    gsm.device.print_devices(options.args)
  File "/usr/lib/python3/dist-packages/gnuradio/gsm/device.py", line 72, in print_devices
    devices = exclude(get_devices(hint))
              ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/gnuradio/gsm/device.py", line 52, in exclude
    return [dev for dev in devices if not match(dev, filters)]
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/gnuradio/gsm/device.py", line 52, in <listcomp>
    return [dev for dev in devices if not match(dev, filters)]
                                          ^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/gnuradio/gsm/device.py", line 45, in match
    if (k not in dev or dev[k] != v):
        ^^^^^^^^^^^^
TypeError: argument of type 'osmosdr.osmosdr_python.device_t' is not iterable

By looking at the error we can see the that issue is that dev is used as a if it was a dict but it's actually a device_t. This pull request adds a helper function to convert a device_t to a dict so the existing code can work as expected.

The conversion is done by using the to_string method which is available on objects of type device_t which can give strings like this: "label='RFSPACE SDR-IQ Receiver',sdr-iq=/dev/ttyUSB0" or this "default_input=False,default_output=False,device_id=3,driver=audio,label='Monitor of USB Audio',soapy=2". Then the string is parsed into pairs of key/value using parse_qsl and finally the values, which are still strings at this point, are evaluated with literal_eval .

[0xxstone]

If we want to avoid the import of parse_qsl, we can also do it this way:

def device_to_dict(dev):
    dev_dict = {}
    for pair in dev.split(","):
        k, v = pair.split("=")
        try:
            dev_dict[k] = literal_eval(v)
        except (ValueError, SyntaxError):
            dev_dict[k] = literal_eval(f"'{v}'")
    return dev_dict

[0xxstone]

This try/except is required to handle the cases where the values returned by to_string are strings without quotes or strings without quotes starting with / like /dev/ttyUSB0 for example.

[0xxstone]

This PR should also fix #636