gh-101266: Fix __sizeof__ for subclasses of int (#101394)

gh-101266: Fix __sizeof__ for subclasses of int (#101394)

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.

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* Issue: gh-101266
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