[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.)

Lib/test/test_long.py

@@ -1596,5 +1596,44 @@ def test_square(self):
                 self.assertEqual(n**2,
                     (1 << (2 * bitlen)) - (1 << (bitlen + 1)) + 1)
 
+    def test___sizeof__(self):
+        self.assertEqual(int.__itemsize__, sys.int_info.sizeof_digit)
+
+        # Pairs (test_value, number of allocated digits)
+        test_values = [
+            # We always allocate space for at least one digit, even for
+            # a value of zero; sys.getsizeof should reflect that.
+            (0, 1),
+            (1, 1),
+            (-1, 1),
+            (BASE-1, 1),
+            (1-BASE, 1),
+            (BASE, 2),
+            (-BASE, 2),
+            (BASE*BASE - 1, 2),
+            (BASE*BASE, 3),
+        ]
+
+        for value, ndigits in test_values:
+            with self.subTest(value):
+                self.assertEqual(
+                    value.__sizeof__(),
+                    int.__basicsize__ + int.__itemsize__ * ndigits
+                )
+
+        # Same test for a subclass of int.
+        class MyInt(int):
+            pass
+
+        self.assertEqual(MyInt.__itemsize__, sys.int_info.sizeof_digit)
+
+        for value, ndigits in test_values:
+            with self.subTest(value):
+                self.assertEqual(
+                    MyInt(value).__sizeof__(),
+                    MyInt.__basicsize__ + MyInt.__itemsize__ * ndigits
+                )
+
+
 if __name__ == "__main__":
     unittest.main()