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
• Pull request title in appropriate format (PEP 123: Mark Final)
• Status changed to Final (and Python-Version is correct)
• Canonical docs/spec linked with a canonical-doc directive (or canonical-pypa-spec, for packaging PEPs)

Helps #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.

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

[hugovk]

@markshannon I assume this is fine, I'll merge it now.