Improved behavior of :class:`asyncio.TaskGroup` when an external cancellation collides with an internal cancellation. For example, when two task groups are nested and both experience an exception in a child task simultaneously, it was possible that the outer task group would misbehave, because its internal cancellation was swallowed by the inner task group.
In the case where a task group is cancelled externally and also must raise an :exc:`ExceptionGroup`, it will now call the parent task's :meth:`~asyncio.Task.cancel` method. This ensures that a :exc:`asyncio.CancelledError` will be raised at the next :keyword:`await`, so the cancellation is not lost.
An added benefit of these changes is that task groups now preserve the cancellation count (:meth:`asyncio.Task.cancelling`).
In order to handle some corner cases, :meth:`asyncio.Task.uncancel` may now
reset the undocumented _must_cancel flag when the cancellation count
reaches zero.