Capture case where gpsd is not yet ready

[renestraub]

When starting too fast, gpsd might no have devices list ready.

root@am335x-nmhw21:~/prj/ubxlib# python3 -m examples.show_version    
2020-08-29 15:44:45,881 INFO     connecting to gpsd
2020-08-29 15:44:45,883 INFO     waiting for receive thread to become active
2020-08-29 15:44:46,887 ERROR    timeout while connecting to device
Traceback (most recent call last):
  File "/usr/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/sysroot/home/root/prj/ubxlib/examples/show_version.py", line 30, in <module>
    res = ubx.poll(poll_version)
  File "/sysroot/home/root/prj/ubxlib/ubxlib/server.py", line 83, in poll
    self.send(message)
  File "/sysroot/home/root/prj/ubxlib/ubxlib/server.py", line 123, in send
    assert self.selected_device
AssertionError