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/bytesio.c b/Modules/_io/bytesio.c
index f643e91..82f3851 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -809,7 +809,7 @@
 {
     Py_ssize_t res;
 
-    res = sizeof(bytesio);
+    res = _PyObject_SIZE(Py_TYPE(self));
     if (self->buf)
         res += self->buf_size;
     return PyLong_FromSsize_t(res);