gh-62912: fix pure-Python operator.index()

[skirpichev]

• Issue: Pure Python operator.index doesn't match the C version. #62912

---

📚 Documentation preview 📚: https://cpython-previews--148023.org.readthedocs.build/

[skirpichev]

CC @serhiy-storchaka

This is based on your patch, especially tests. But I believe that this simplified version is now ok. Strictly speaking, new docs lie for some broken int's subclasses like:

class BadInt(int):
    def __index__(self):
        return 42

We need to fix also concat/iconcat. But I would prefer to do this in a separate PR.