Fixed #23609 -- Changed behavior when dropping a NULL constraint to update rows with NULL to default

Ticket: https://code.djangoproject.com/ticket/23609
It's the right way to support Python 2 and Python 3. See [PEP 3105](http://legacy.python.org/dev/peps/pep-3105/). I mean, I can drop it again, but I don't see the reason for that.