Fix weakref callback errors during finalization

[RazerM]

Using blinker 1.6.3, I get a lot of errors like the following when invoking the Flask CLI, running tests, etc.

Exception ignored in: <bound method Signal._cleanup_receiver of <blinker.base.NamedSignal object at 0x10a67ef90; 'request-started'>>
Traceback (most recent call last):
  File "/path/to/venv/lib/python3.7/site-packages/blinker/base.py", line 430, in _cleanup_receiver
TypeError: 'NoneType' object is not callable

The only issue I could find about this is one made for sentry: getsentry/sentry-python#2183

I found that the cause is on this line:

blinker/src/blinker/base.py

Line 430 in 36560da

self._disconnect(t.cast(IdentityType, receiver_ref.receiver_id), ANY_ID)

During cleanup, t.cast is None.

weakref finalization callbacks need to follow __del__ rules, i.e.
any globals referenced may have had their attributes set to None.

[pgjones]

Thanks, I'd be tempted to drop the usage of cast and simply type: ignore the lines in question. Do you think the cast usage is better?

Edit: I'll merge this - you've done the work and I don't have a strong view.

[RazerM]

That was my first instinct, then I thought that type safety is good since exceptions here may be unraisable. I also don't have a strong view either way, but I think the reference to cast is safe.

Thanks for merging :-)