PEP 689: Tweak a policy, add canonical-doc and Mark as Final

[encukou]

Mark Final

• 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
  • I've linked this PR in the Steering Council request
• Pull request title in appropriate format (PEP 123: Mark Final)
  • I hope it is!
• Status changed to Final (and Python-Version is correct)
• Canonical docs/spec linked with a canonical-doc directive (or pypa-spec, for packaging PEPs)

Change an existing PEP

• Change is either:
  • To a Draft PEP
  • To an Accepted or Final PEP, with Steering Council approval
    • I've linked this PR in the Steering Council request
  • To fix an editorial issue (markup, typo, link, header, etc)
• PR title prefixed with PEP number (e.g. PEP 123: Summary of changes)

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📚 Documentation preview 📚: https://pep-previews--3123.org.readthedocs.build/