title: Semantics of `async for` - can __anext__ calls overlap?
body: <p>If <code>__anext__</code> gives control back to event loop (via an <code>await</code>, or <code>call_soon</code>/<code>call_later</code>), is it possible for another <code>__anext__</code> to be called on the same instance, while first one had not resolved yet, or they would be queued? Are there any other cases where it would be unsafe to assume that only one <code>__anext__</code> is running at the same time?</p>