I am currently working on a patch that allows large ranges and large
indices. The trick is to define tp_as_mapping->mp_subscript.
Then range_item() is rarely used, only by functions calling directly the
PySequence_* functions, instead of the abstract PyObject_*.

There is still a limit with len(), which seems bound by the size_t limit.
Most of the tests in test_builtin were re-enabled.

I join the current version of the patch.
I'm still working on further simplifications, and maybe supporting
slices on ranges...

Note: I found more useful to store a "range->end" member, which is the
multiple of "step" just beyond the "stop" limit.