Fix yield from not working on non-iterator iterables inside async generators

[johnslavik]

Previously, this wouldn't work:

import asyncio

async def agen():
    yield from range(10)

async def main():
    [x async for x in agen()]  # ho ho is this a Prime reference

asyncio.run(main())

PyObject_GetIter increments iterable's refcount so I think we're game?