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.