Fix aiterator() signature.

[john-parton]

Queryset.aiterator() was set up to by annotated as async () -> Iterator which implies the call semantics are

for obj in await queryset.aiterator():
    ...

But what we actually want is

async for obj in queryset.aiterator():
   ...

Updated signature to match Django

[john-parton]

Rebased/force-pushed to avoid warning from CI

[sbdchd]

Thanks!