cannot froze a subclass

To continue on my previous issue, I would like to froze a sublcass but I can't manage to make it work. As you manage to make it work in `BoxList` what am I missing ?

```python
class toto(Box):

def __init__(self, valeur):

super(Box, self).__init__({"valeur": {"valeur": valeur}}, frozen_box=True)

truc = toto("truc")
```

```python
truc.valeur = "toto"
```