PEP 590: Mark as Final

[hugovk]

• [ ] Final implementation has been merged (including tests and docs)
• [ ] PEP matches the final implementation
• [ ] Any substantial changes since the accepted version approved by the SC/PEP delegate
• [x] Pull request title in appropriate format (PEP 123: Mark Final)
• [x] Status changed to Final (and Python-Version is correct)
• [x] Canonical docs/spec linked with a canonical-doc directive (or canonical-pypa-spec, for packaging PEPs)

Helps https://github.com/python/peps/issues/2872.

This is still listed under "Accepted PEPs (accepted; may not be implemented yet)".

If it's fully implemented, let's mark it as Final and add a banner linking to the canonical docs at https://docs.python.org/3/c-api/call.html#vectorcall.

Also remove a redundant header and footer.

---

📚 Documentation preview 📚: https://pep-previews--3598.org.readthedocs.build/