PEP 780: Update to use sys.abi_info

[zklaus]

• Change is either:
  • To a Draft PEP
  • To an Accepted or Final PEP, with Steering Council approval
  • To fix an editorial issue (markup, typo, link, header, etc)
• PR title prefixed with PEP number (e.g. PEP 123: Summary of changes)

This PR updates PEP 780 to make use of the new sys.abi_info namespace that stems from the original PEP 780 discussion.

Pinging @rgommers, @lysnikolaou.

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📚 Documentation preview 📚: https://pep-previews--4608.org.readthedocs.build/

[rgommers]

Thanks @zklaus! On reflection, I think I like the name::value syntax, it seems better than using things like sys_abi_info_free_threaded as boolean markers.

Only two minor comments from me.

[mgorny]

LGTM. Thanks for addressing my feedback.