bpo-46771: cancel counts

[Tinche]

Changes:

• Task._cancel_requested: bool is now Task._num_cancels_requested: int
• Task.cancelling now returns an int
• Task.uncancel also returns an int, the number of remaining cancel requests
• the CancelScope no longer cancels with a nonce
• the CancelScope uncancels in __exit__, and propagates the CancelError if there are more cancel requests
• CancelScope.cancelled is now a property
• added a couple of tests, not final

I have not updated any of the comments or docs yet.

https://bugs.python.org/issue46771

[asvetlov]

I like it!
Please feel free to press 'Merge' when you are ready.

[Tinche]

@asvetlov this is a competing proposal to using the _COUNTERS dict, so maybe we should discuss this further? This makes the cancel count be on the task, instead of in the dict.

[asvetlov]

I understand.
I have a feeling that your approach is better, it doesn't throw TimeoutError if the task was explicitly cancelled.

[Tinche]

@asvetlov Cool, but you'll need to merge this :)

[bedevere-bot]

@asvetlov: Please replace # with GH- in the commit message next time. Thanks!

[asvetlov]

Done. Thanks!