PEP 646: 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 #3579 and #2872.

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📚 Documentation preview 📚: https://pep-previews--3671.org.readthedocs.build/

[JelleZijlstra]

PEP 646 actually also made some grammar changes that as far as I can tell aren't documented in the language reference. I'll open an issue on CPython.

[hugovk]

@python/organization-owners Please block @raea3225 from the org, they're repeatedly posting time-wasting comments. I've already reported them to GitHub.