signal usage in get_raw/get_json incompatible with threading

[klauer]

The last thing I had on the docket here was to try and toss the archapp calls into a thread, in order to free up the gui while PV's were being added to the archive viewer. I tried a few things, but ended up hitting upon the fact that python's signal only works in the main thread of the main interpreter. 😞

traceback

Traceback (most recent call last):
  File "/cds/home/r/roberttk/devrepos/atef/atef/qt_helpers.py", line 332, in run
    self.return_value = self.func(*self.args, **self.kwargs)
  File "/cds/home/r/roberttk/devrepos/atef/atef/widgets/config/data.py", line 267, in add_sig
    widget.add_signal(pv, dev_attr=dev_attr, update_curves=False)
  File "/cds/home/r/roberttk/devrepos/atef/atef/widgets/archive_viewer.py", line 314, in add_signal
    data = self.get_pv_data_snippet(pv)
  File "/cds/home/r/roberttk/devrepos/atef/atef/widgets/archive_viewer.py", line 348, in get_pv_data_snippet
    data = self.archapp._data.get_raw(pv, prev, today)
  File "/cds/home/r/roberttk/devrepos/archapp/archapp/data.py", line 166, in get_raw
    data = get_json(url)
  File "/cds/home/r/roberttk/devrepos/archapp/archapp/url.py", line 70, in get_json
    signal.signal(signal.SIGALRM, _raise_timeout)
  File "/cds/group/pcds/pyps/conda/py39/envs/pcds-5.5.1/lib/python3.9/signal.py", line 56, in signal
    handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
ValueError: signal only works in main thread of the main interpreter

perhaps a deeper rework of archapp could help, but at least for now caching should make this less painful on repeat archive-viewer openings?

Originally posted by @tangkong in pcdshub/atef#132 (comment)

[ZLLentz]

signal is used here entirely to manage timeouts. There are almost certainly better ways to handle this that can be implemented here.