some Pyston class use hidden class to store their attributes,
instead use PyDictObejct. This can get some speedup. But we can't
get a writable dict pointer if a class use hidden class.

So if code want to copy the dict, use PyObject_GetDict, if code want
to add new item to it, use PyObject_SetDict.

In Pyston, PyClassObject is just an opaque pointer. Don't access it directly.
Use PyObject_GetAttrWrapper which provided by Pyston:

-        d = ((PyClassObject *)cls)->cl_dict;
+        d = PyObject_GetAttrWrapper(cls);