Here comes a patch, changing the behaviour to:

./python -q
>>> class C:
...   pass
... 
>>> (1).__class__ = 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __class__ must be set to a class defined by a class statement, not 'int' object
>>> (1).__class__ = object
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: class__ must be set to a class defined by a class statement, not 'object'
>>> (1).__class__ = C
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __class__ assignment: only for instances of classes defined by class statements