Freeze with "cannot schedule new futures after shutdown"

On Debian Bullseye, with python 3.9.2, electrum 4.1.5 (running from sources) can freeze with "cannot schedule new futures after shutdown" logged repeatedly. QT UI became unresponsive and I had to kill the process manually. I hit this while trying to create a hardware wallet (see below for more detail):

Traceback (most recent call last):
  File "/home/user/src/github.com/spesmilo/electrum/electrum/util.py", line 1120, in wrapper
    return await func(*args, **kwargs)
  File "/home/user/src/github.com/spesmilo/electrum/electrum/interface.py", line 487, in wrapper_func
    return await func(self, *args, **kwargs)
  File "/home/user/src/github.com/spesmilo/electrum/electrum/interface.py", line 510, in run
    await self.open_session(ssl_context)
  File "/home/user/src/github.com/spesmilo/electrum/electrum/interface.py", line 657, in open_session
    await group.spawn(self.monitor_connection)
  File "/home/user/.local/lib/python3.9/site-packages/aiorpcx/curio.py", line 255, in __aexit__
    await self.join()
  File "/home/user/.local/lib/python3.9/site-packages/aiorpcx/curio.py", line 213, in join
    raise task.exception()
  File "/home/user/src/github.com/spesmilo/electrum/electrum/synchronizer.py", line 83, in _run_tasks
    await group.spawn(self.main())
  File "/home/user/.local/lib/python3.9/site-packages/aiorpcx/curio.py", line 255, in __aexit__
    await self.join()
  File "/home/user/.local/lib/python3.9/site-packages/aiorpcx/curio.py", line 213, in join
    raise task.exception()
  File "/home/user/src/github.com/spesmilo/electrum/electrum/synchronizer.py", line 263, in main
    await run_in_thread(self.wallet.synchronize)
  File "/home/user/.local/lib/python3.9/site-packages/aiorpcx/curio.py", line 68, in run_in_thread
    return await get_event_loop().run_in_executor(None, func, *args)
  File "/usr/lib/python3.9/asyncio/base_events.py", line 814, in run_in_executor
    executor.submit(func, *args), loop=self)
  File "/usr/lib/python3.9/concurrent/futures/thread.py", line 161, in submit
    raise RuntimeError('cannot schedule new futures after shutdown')
RuntimeError: cannot schedule new futures after shutdown

By adding some debug logging to util.py I was able to find that the app was trying to display the following (masked by the other error):

Unable to reference hid.device; maybe hid package is masking hidapi? Try:
    pip3 uninstall hid

This hint was correct, and by uninstalling hid the freeze went away. I include below remaining steps I had to take from stock Debian to create a Trezor wallet successfully, in case there's some improvements that could be made.

Next issue was the version of the trezor pulled in by pip3 install -r contrib/requirements/requirements-hw.txt:

W | plugins.trezor.qt.Plugin | Library version for 'trezor' is incompatible.
Installed: 0.13.0, Needed: 0.12.0 <= x < 0.13

This was fixed by editing requirements-hw.txt to set trezor[hidapi]>=0.12.0,<0.13 and reinstalling the older trezor package.

Start with the exception path in electrum/util.py, electrum/interface.py, and electrum/synchronizer.py, then inspect contrib/requirements/requirements-hw.txt and the Trezor plugin mentioned in the report. Reproduce the Debian/Python environment with the conflicting hid and trezor packages; done means the hardware-wallet flow does not freeze and the supported dependency versions are handled clearly.