Use PEP's versions API

[StanFromIreland]

https://peps.python.org/api/release-cycle.json

This is slightly unfortunate, since we need to pull the devguide anyway.

[m-aciek]

For the reference:

This file is now generated and published from the PEPs repo (python/peps#4331) and will be removed from the devguide soon (python/devguide#1685).