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/Modules/_io/bufferedio.c b/Modules/_io/bufferedio.c
index 29e000b..6bb2200 100644
--- a/Modules/_io/bufferedio.c
+++ b/Modules/_io/bufferedio.c
@@ -423,7 +423,7 @@
 {
     Py_ssize_t res;
 
-    res = sizeof(buffered);
+    res = _PyObject_SIZE(Py_TYPE(self));
     if (self->buffer)
         res += self->buffer_size;
     return PyLong_FromSsize_t(res);
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index 31cc1f7..eef3b3d 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -991,7 +991,7 @@
 {
     Py_ssize_t res;
 
-    res = sizeof(bytesio);
+    res = _PyObject_SIZE(Py_TYPE(self));
     if (self->buf && !SHARED_BUF(self))
         res += _PySys_GetSizeOf(self->buf);
     return PyLong_FromSsize_t(res);