Currently PyNumber_Index() can return something that's an instance of a strict subclass of int. For example PyNumber_Index(Py_True) returns Py_True. The same for operator.index():

>>> import operator
>>> operator.index(True)
True

The proposed PR makes it always return an int.

To avoid possible overhead for creating temporary integer object, added private function _PyNumber_Index() with the past behavior. It can be used for short-living integer objects which for which only its value will be used, but not its methods. For example in the implementation of PyLong_AsLong() and similar functions.

See also issue17576.