PEP 673: 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,
  or canonical-typing-spec for typing PEPs)

Helps #2872 and #3579.

Documented in the typing spec at https://typing.readthedocs.io/en/latest/spec/generics.html#self

---

📚 Documentation preview 📚: https://pep-previews--3644.org.readthedocs.build/