PyNumber_Index() now always returns an instance of int.

- If the argument is a direct int then return it.
- If it is a subclass of int then return a direct int copy.
- Otherwise call type(obj).__index__(obj)
- If a direct int, return it
- If a subclass of int, raise a DeprecationWarning and return a direct int copy
- If not an int, raise TypeError

If we go in this direction we should add a DeprecationWarning for __str__() returning not direct str. I am not sure that it is right. It adds a burden on authors of special methods to always convert the result to the corresponding direct type, while this conversion can silently (and more efficiently) be performed in the interpreter core.