Handle the return of non-integer value in __index__ function

[HyeockJinKim]

An error should occur when returning a non-integer value from index.

Expected result (cpython)

>>> class C:
... def __index__(self):
... return 'a'
... 
>>> c = C()
>>> a = [1, 2 ,4]
>>> a[c]
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: __index__ returned non-int (type str)

Actual result (gpython)

>>> class C:
... def __index__(self):
... return 'a'
... 
>>> c = C()
>>> a = [1, 2, 4]
>>> a[c]
1

[ncw]

Well spotted :-)