[issue46771] Add some form of cancel scopes

[Alex Grönholm]

Alex Grönholm <alex.gronholm at nextday.fi> added the comment:

> Propagating an ExceptionGroup where every exception can be inspected to see if it was caused by this code or not still seems like the safe option to me (and obviously still has the cancel count implicitly).

Note that this, too, causes backwards incompatible changes in cancellation behavior. Previously, when the task was cancelled twice, only one CancelledError was raised. Now it would raise a BaseExceptionGroup instead.

The current backward incompatible changes in cancellation behavior are already causing 10 tests to fail in the AnyIO test suite. I'm trying to find an alternate solution that does not break anything.

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<https://bugs.python.org/issue46771>
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