Port test_asyncgen to Python 3.10 (GH-4044)

The loop parameter of asyncio.sleep(), deprecated in Python 3.8, has
been removed in Python 3.10. asyncio.sleep() gets implicitly the loop
in Python 3.10 and it is safe to do the same on Python 3.6:
https://docs.python.org/dev/whatsnew/3.10.html#changes-in-the-python-api