That change fixes that bug, but I think there may be another bug involving when a custom async iterator is passed rather than an async generator. This is at the limit of my knowledge, so any guidance would be appreciated. The test I wrote in the PR currently fails, due to some `tp_iternext` slot being NULL sometimes. Maybe different cases are needed for Coroutine/non-Coroutine?

But it definitely seems like the aiter()/anext() code needs more test coverage.