You're not testing `models.QuerySet.__setstate__` at all here.

Override `__getstate__` instead:

```python
class PreviousDjangoVersionQuerySet(models.QuerySet):
    def __getstate__(self):
        state = super(models.QuerySet, self).__getstate__()
        state['django_version'] = str(float(state['django_version']) - 1)  # previous major version
        return state
```