Fix aiterator() signature. (#262)

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

django-stubs/db/models/query.pyi

@@ -42,7 +42,7 @@ class _BaseQuerySet(Generic[_T], Sized):
     def __and__(self, other: _BaseQuerySet[_T]) -> Self: ...
     def __or__(self, other: _BaseQuerySet[_T]) -> Self: ...
     def iterator(self, chunk_size: int = ...) -> Iterator[_T]: ...
-    async def aiterator(self, chunk_size: int = ...) -> Iterator[_T]: ...
+    def aiterator(self, chunk_size: int = ...) -> AsyncIterator[_T]: ...
     def aggregate(self, *args: Any, **kwargs: Any) -> dict[str, Any]: ...
     async def aaggregate(self, *args: Any, **kwargs: Any) -> dict[str, Any]: ...
     def get(self, *args: Any, **kwargs: Any) -> _T: ...