PEP 560: 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 and #3579.

PEP 560 is listed under "Accepted PEPs (accepted; may not be implemented yet)" but was implemented in Python 3.7:

• https://docs.python.org/3/whatsnew/3.7.html#pep-560-core-support-for-typing-module-and-generic-types
• https://docs.python.org/3/library/types.html#types.resolve_bases
• https://docs.python.org/3/library/types.html#types.get_original_bases

Let's mark it as Final and link to the docs.

Also remove redundant Content-Type header and Local Variables footer.

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📚 Documentation preview 📚: https://pep-previews--3570.org.readthedocs.build/pep-0560.html