Django-virtual-models

- Add support to nested prefetch lookups like `v.VirtualModel(manager=User.objects, lookup="course__facilitators")`
* Warning: this will remain undocumented for now, because the behavior is strange:
the prefetch is made inside `course` in this case, due to Django behavior.
- Add parameter `serializer_context` to be used in `v.Annotation` and `get_prefetch_queryset`.

- Fix support for custom manager in `VirtualModel` initialization
- Separate method for `_build_prefetch` to allow overrides

- More robust `is_preloaded` check
- Add Django 4.2 to tests.

Add Python 3.11 to tests.

- README update.

- Avoid redundant `to_attr` when nesting Virtual Models
- Refactor `fields.py` to use `self.parent` and `self.field_name`
- Simplify docs and tests by not using manager param