Oh, and to explain my negative comment from earlier: that was my reaction when I realised I also needed to write PyLong versions of _PyEval_SliceIndex and PySlice_GetIndicesEx to make range slicing with large integers work properly.

As it turned out, the end result wasn't as scary as I initially feared (while compute_slice_indices is quite long, most of that is just the verbosity of PyLong arithmetic).