Retry connection to callink API after read timeout

[emmatyping]

It seems that callink will (not infrequently, but rarely enough...) timeout on trying to connect to their API endpoint. We should retry the connection. The error looks something like:

.... more traceback above

Traceback (most recent call last):
  File "/opt/share/utils/sbin/signat", line 212, in <module>
    main()
  File "/opt/share/utils/sbin/signat", line 204, in main
    show_by_username(args.username)
  File "/opt/share/utils/sbin/signat", line 49, in show_by_username
    show_groups_by_student_signat(uid)
  File "/opt/share/utils/sbin/signat", line 97, in show_groups_by_student_signat
    groups = groups_by_student_signat(uid)
  File "/usr/lib/python3/dist-packages/ocflib/ucb/groups.py", line 135, in groups_by_student_signat
    return _get_osl({'UID': uid}, service, parser)
  File "/usr/lib/python3/dist-packages/ocflib/ucb/groups.py", line 150, in _get_osl
    r = requests.get(url, timeout=20)
  File "/usr/lib/python3/dist-packages/requests/api.py", line 75, in get
    return request('get', url, params=params, **kwargs)
  File "/usr/lib/python3/dist-packages/requests/api.py", line 60, in request
    return session.request(method=method, url=url, **kwargs)
  File "/usr/lib/python3/dist-packages/requests/sessions.py", line 533, in request
    resp = self.send(prep, **send_kwargs)
  File "/usr/lib/python3/dist-packages/requests/sessions.py", line 646, in send
    r = adapter.send(request, **kwargs)
  File "/usr/lib/python3/dist-packages/requests/adapters.py", line 529, in send
    raise ReadTimeout(e, request=request)
requests.exceptions.ReadTimeout: HTTPSConnectionPool(host='studentgroupservice.sait-west.berkeley.edu', port=443): Readtimed out. (read timeout=20)

[ben9583]

This also happens if you try to run signat oid [oid] where oid is greater than 2^31-1. Obviously, there are no organizations beyond here, but it may be good to just handle the timeout as opposed to throwing the python exception.