PEP 800: Add Discussions-To thread to Post-History

[brianschubert]

I think these are usually updated together, but Post-History wasn't synced in #4508.

Add trailing comma since that seems to be the preferred style in other PEPs.

cc @JelleZijlstra @hugovk

• 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)

---

📚 Documentation preview 📚: https://pep-previews--4874.org.readthedocs.build/