async def test_exception_refcycles_parent_task(self):
        """Test that TaskGroup deletes self._parent_task"""
        tg = asyncio.TaskGroup()
        exc = None
        class _Done(Exception):
            pass
        async def coro_fn():
            async with tg:
                raise _Done
        try:
            async with asyncio.TaskGroup() as tg2:
                tg2.create_task(coro_fn())
        except* _Done as excs:
            exc = excs.exceptions[0].exceptions[0]
        self.assertIsInstance(exc, _Done)
        self.assertListEqual(gc.get_referrers(exc), no_other_refs())