PEP 709: 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.

PEP 709 was implemented in Python 3.12, we should mark it as Final so it's no longer listed under Accepted PEPs (accepted; may not be implemented yet).

Should this have a .. canonical-doc:: banner at the top to link to canonical docs?

For example, compare https://peps.python.org/pep-0634/

All I could find in the CPython docs is the What's New entry, but perhaps as an internal change, "canonical docs" doesn't apply in this instance?

---

📚 Documentation preview 📚: https://pep-previews--3591.org.readthedocs.build/

[carljm]

I don't think canonical docs applies, since it's an internal implementation detail with only a few user-visible edge cases.

Thanks for doing this update!