`anyio>=4.0` breaks our backend implementations

[Archmonger]

Current Situation

Looks like some changes to the latest versions of anyio fundamentally breaks how we currently spawn our backends.

Exception in thread Thread-3 (run_dispatcher):
Traceback (most recent call last):
  File "C:\Users\Markg\Documents\Repositories\_testbed\.venv\lib\site-packages\reactpy\backend\flask.py", line 225, in main
    await serve_layout(
  File "C:\Users\Markg\Documents\Repositories\_testbed\.venv\lib\site-packages\reactpy\core\serve.py", line 42, in serve_layout
    async with create_task_group() as task_group:
  File "C:\Users\Markg\Documents\Repositories\_testbed\.venv\lib\site-packages\anyio\_backends\_asyncio.py", line 668, in __aexit__
    raise cancelled_exc_while_waiting_tasks
  File "C:\Users\Markg\Documents\Repositories\_testbed\.venv\lib\site-packages\anyio\_backends\_asyncio.py", line 648, in __aexit__
    await asyncio.wait(self.cancel_scope._tasks)
  File "C:\Users\Markg\AppData\Local\Programs\Python\Python310\lib\asyncio\tasks.py", line 384, in wait
    return await _wait(fs, timeout, return_when, loop)
  File "C:\Users\Markg\AppData\Local\Programs\Python\Python310\lib\asyncio\tasks.py", line 491, in _wait
    await waiter
asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\Markg\AppData\Local\Programs\Python\Python310\lib\threading.py", line 1016, in _bootstrap_inner
    self.run()
  File "C:\Users\Markg\AppData\Local\Programs\Python\Python310\lib\threading.py", line 953, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Users\Markg\Documents\Repositories\_testbed\.venv\lib\site-packages\flask\ctx.py", line 181, in wrapper
    return reqctx.app.ensure_sync(f)(*args, **kwargs)
  File "C:\Users\Markg\Documents\Repositories\_testbed\.venv\lib\site-packages\reactpy\backend\flask.py", line 253, in run_dispatcher
    loop.run_until_complete(main_future)
  File "C:\Users\Markg\AppData\Local\Programs\Python\Python310\lib\asyncio\base_events.py", line 649, in run_until_complete
    return future.result()
asyncio.exceptions.CancelledError

Proposed Actions

Create compatibility for anyio>=4.0, or limit anyio to <4.0.

[Archmonger]

No longer needed due to #1113