D4118: index: make node tree a Python object

[Yuya Nishihara]

> +static int nt_init_py(nodetree *self, PyObject *args)
> +{
> +	PyObject *index;
> +	unsigned capacity;
> +	if (!PyArg_ParseTuple(args, "OI", &index, &capacity))

"O!I" to make sure index is an index object.