PEP 751: Add Discussions-To and Post-History

Basic requirements (all PEP Types)

[x] Read and followed PEP 1 & PEP 12
[x] File created from the latest PEP template
[x] PEP has next available number, & set in filename (pep-NNNN.rst), PR title (PEP 123: <Title of PEP>) and PEP header
[x] Title clearly, accurately and concisely describes the content in 79 characters or less
[x] Core dev/PEP editor listed as Author or Sponsor, and formally confirmed their approval
[x] Author, Status (Draft), Type and Created headers filled out correctly
[x] PEP-Delegate, Topic, Requires and Replaces headers completed if appropriate
[x] Required sections included
- [x] Abstract (first section)
- [x] Copyright (last section; exact wording from template required)
[x] Code is well-formatted (PEP 7/PEP 8) and is in code blocks, with the right lexer names if non-Python
[x] PEP builds with no warnings, pre-commit checks pass and content displays as intended in the rendered HTML
[x] Authors/sponsor added to .github/CODEOWNERS for the PEP

[x] PEP topic discussed in a suitable venue with general agreement that a PEP is appropriate
[x] Suggested sections included (unless not applicable)
- [ ] Motivation
- [ ] Rationale
- [ ] Specification
- [ ] Backwards Compatibility
- [ ] Security Implications
- [ ] How to Teach This
- [ ] Reference Implementation
- [ ] Rejected Ideas
- [ ] Open Issues
[x] Python-Version set to valid (pre-beta) future Python version, if relevant
[x] Any project stated in the PEP as supporting/endorsing/benefiting from the PEP formally confirmed such
[x] Right before or after initial merging, PEP discussion thread created and linked to in Discussions-To and Post-History