Fix for SF bug 516727: MyInt(2) + "3" -> NotImplemented
Fix for SF bug 516727: MyInt(2) + "3" -> NotImplemented

PyNumber_Add() tries the nb_add slot first, then falls back to
sq_concat.  However, tt didn't check the return value of sq_concat.
If sq_concat returns NotImplemented, raise the standard TypeError.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator