Issue #25421: __sizeof__ methods of builtin types now use dynamic basic size.
This allows sys.getsize() to work correctly with their subclasses with
__slots__ defined.
diff --git a/Objects/setobject.c b/Objects/setobject.c
index 704d7e2..2faaf12 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -1940,7 +1940,7 @@
 {
     Py_ssize_t res;
 
-    res = sizeof(PySetObject);
+    res = _PyObject_SIZE(Py_TYPE(so));
     if (so->table != so->smalltable)
         res = res + (so->mask + 1) * sizeof(setentry);
     return PyLong_FromSsize_t(res);