PEP 581: 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, which says:

PEP-581 Using GitHub Issues for CPython: Since the PEP's content is tightly focused on the rationale for choosing GitHub rather than the current, active process for using it and is thus now of historical rather than procedural value, it should be marked Final (rather than Active) like other transition-oriented Process PEPs have.

Also remove some redundant fields.

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📚 Documentation preview 📚: https://pep-previews--3807.org.readthedocs.build/

[Mariatta]

Thanks