gh-116720: Fix corner cases of taskgroups (#117407)

This prevents external cancellations of a task group's parent task to
be dropped when an internal cancellation happens at the same time.
Also strengthen the semantics of uncancel() to clear self._must_cancel
when the cancellation count reaches zero.

Co-Authored-By: Tin Tvrtković <tinchester@gmail.com>
Co-Authored-By: Arthur Tacca