[CI] fix test_aio_poll_no_leaks for python 3.6 (#29327)
[CI] fix test_aio_poll_no_leaks for python 3.6 (#29327)

asyncio.all_tasks() doesn't exist in python 3.6.
ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray