Hum, another function should be updated. Do you want to propose a fix Erlend?

static int
type_set_annotations(PyTypeObject *type, PyObject *value, void *context)
{
    if (!(type->tp_flags & Py_TPFLAGS_HEAPTYPE)) {
        PyErr_Format(PyExc_TypeError, "can't set attributes of built-in/extension type '%s'", type->tp_name);
        return -1;
    }