Return a new slice object with the given values. The start,
stop, and step parameters are used as the values of the
slice object attributes of the same names. Any of the values may be
NULL, in which case the None will be used for the
corresponding attribute. Returns NULL if the new object could
not be allocated.
int PySlice_GetIndices(PySliceObject *slice, int length,
int *start, int *stop, int *step)