PEP 2026: Calendar versioning for Python

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