bpo-30354: Update data model documentation for super() (GH-1561)

bpo-30354: Update data model documentation for super() (GH-1561)

The data model section of the language reference was written well
before the zero-argument form of super() was added.

To avoid giving the impression that they're doing something
unusual, this updates the description of `__new__` and `__init__`
to use the zero-argument form.

Patch by Cheryl Sabella.