Django-cte

- Python 3 compatibility.

- Fix CTE alias bug.

- Use `_default_manager` instead of `objects`.

- Fix recursive CTE pickling. Note: this is currently [broken on Django
master](https://github.com/django/django/pull/9134#pullrequestreview-112057277).

- `With.queryset()` now uses the CTE model's manager to create a new `QuerySet`,
which makes it easier to work with custom `QuerySet` classes.

- BACKWARD INCOMPATIBLE CHANGE: `With.queryset()` no longer accepts a `model`
argument.
- Improve `With.queryset()` to select directly from the CTE rather than
joining to anoter QuerySet.
- Refactor `With.join()` to use real JOIN clause.