[3.11] gh-101266: Fix __sizeof__ for subclasses of int (GH-101394) (#101579)

Fix the behaviour of the `__sizeof__` method (and hence the results returned
by `sys.getsizeof`) for subclasses of `int`. Previously, `int` subclasses gave
identical results to the `int` base class, ignoring the presence of the instance
dictionary.

(Manual backport of #101394 to the Python 3.11 branch.)