examples/web/async.py: type hint error

[otaku42]

Running examples/web/async.py with Python 3.9 (Debian) results in:

(v4l2py-testing) ┌─[✗]─[mrenzmann@parrot]─[~/src/v4l2py-testing/examples/web]
└──╼ $uvicorn async:app
Traceback (most recent call last):
  File "/home/mrenzmann/src/v4l2py-testing/.venv/bin/uvicorn", line 8, in <module>
    sys.exit(main())
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/uvicorn/main.py", line 410, in main
    run(
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/uvicorn/main.py", line 578, in run
    server.run()
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/uvicorn/server.py", line 61, in run
    return asyncio.run(self.serve(sockets=sockets))
  File "/usr/lib/python3.9/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/usr/lib/python3.9/asyncio/base_events.py", line 642, in run_until_complete
    return future.result()
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/uvicorn/server.py", line 68, in serve
    config.load()
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/uvicorn/config.py", line 473, in load
    self.loaded_app = import_from_string(self.app)
  File "/home/mrenzmann/src/v4l2py-testing/.venv/lib/python3.9/site-packages/uvicorn/importer.py", line 21, in import_from_string
    module = importlib.import_module(module_str)
  File "/usr/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 790, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "/home/mrenzmann/src/v4l2py-testing/examples/web/async.py", line 36, in <module>
    class Camera(BaseCamera):
  File "/home/mrenzmann/src/v4l2py-testing/examples/web/async.py", line 42, in Camera
    async def get_clients(self, timeout=None) -> None | list[asyncio.Queue]:
TypeError: unsupported operand type(s) for |: 'NoneType' and 'types.GenericAlias'
(v4l2py-testing) ┌─[✗]─[mrenzmann@parrot]─[~/src/v4l2py-testing/examples/web]

The lazy fix is to remove the return value type hint expression, but there certainly is a better solution ;-)