PEP 578: 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)

Fixes#2872.

Also:

• Fix broken licence link. http://www.opencontent.org/openpub/ has a 500 internal server error. Looks like the whole site is down. https://web.archive.org/web/20190228180246/https://www.opencontent.org/openpub/ is an archive from around the time the PEP was published, and I've verified the body text matches https://spdx.org/licenses/OPUBL-1.0.html (other than whitespace) which I've used as a replacement.
• Update a couple of redirecting links.
• Replace some non-breaking spaces with real spaces.

This is the last unchecked PEP in #2872. Let's close it now, and use new issues such as #3579 and #3781 for tracking the remainder.

---

📚 Documentation preview 📚: https://pep-previews--3811.org.readthedocs.build/