PEP 615: 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 615 was implemented back in Python 3.9.

Let's mark it as Final, add a banner link to the canonical docs at https://docs.python.org/3/library/zoneinfo.html, and remove a redundant header and footer.

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📚 Documentation preview 📚: https://pep-previews--3592.org.readthedocs.build/

[hugovk]

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