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.

<!-- gh-issue-number: gh-101266 -->
* Issue: gh-101266
<!-- /gh-issue-number -->

Objects/boolobject.c

@@ -4,6 +4,8 @@
 #include "pycore_object.h"      // _Py_FatalRefcountError()
 #include "pycore_runtime.h"       // _Py_ID()
 
+#include <stddef.h>
+
 /* We define bool_repr to return "False" or "True" */
 
 static PyObject *
@@ -153,8 +155,8 @@ bool_dealloc(PyObject* Py_UNUSED(ignore))
 PyTypeObject PyBool_Type = {
     PyVarObject_HEAD_INIT(&PyType_Type, 0)
     "bool",
-    sizeof(struct _longobject),
-    0,
+    offsetof(struct _longobject, long_value.ob_digit),  /* tp_basicsize */
+    sizeof(digit),                              /* tp_itemsize */
     bool_dealloc,                               /* tp_dealloc */
     0,                                          /* tp_vectorcall_offset */
     0,                                          /* tp_getattr */