The error messages are different now:

>>> class C: pass
...
>>> c = C()
>>> c.__class__ = 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __class__ must be set to a class, not 'int' object
>>> c.__class__ = object
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __class__ assignment only supported for mutable types or ModuleType subclasses
>>>