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/dictobject.c b/Objects/dictobject.c
index 50afa3f..b281948 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -2152,7 +2152,7 @@
 {
     Py_ssize_t res;
 
-    res = sizeof(PyDictObject);
+    res = _PyObject_SIZE(Py_TYPE(mp));
     if (mp->ma_table != mp->ma_smalltable)
         res = res + (mp->ma_mask + 1) * sizeof(PyDictEntry);
     return PyInt_FromSsize_t(res);