Support prefetching in `iterator()`

See [Django 4.1 release notes](https://docs.djangoproject.com/en/dev/releases/4.1/#models):

> [`QuerySet.iterator()`](https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.iterator) now supports prefetching related objects as long as the `chunk_size` argument is provided. In older versions, no prefetching was done.