To do `GROUP_BY` queries in django requires one to use `.values()`
eg this groups posts by user getting a count of posts per user.

```
Posts.objects.order_by('user').values('user').annotate(post_count=Count('post'))
```

This would produce a value queryset which serializes its result
objects as dictionaries while `CursorPagination` requires a queryset
with result objects that are model instances.

This commit enables cursor pagination for value querysets.

- had to mangle the tests a bit to test it out. They might need
  some refactoring.
- tried the same for `.values_list()` but it turned out to be
  trickier than I expected since you have to use tuple indexes.