PEP 617: 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.

Also replace some old BPO links with their more accessible GitHub issue equivalents, and update some code block formatting.

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📚 Documentation preview 📚: https://pep-previews--3596.org.readthedocs.build/

[lysnikolaou]

LGTM!