Dynamically substitute main version in versions.rst from release-cycle.json

[corneliusroemer]

In #1582 @hugovk suggested programmatically setting the version of the main branch to avoid having to do it manually each year:

Rather than manually bump this each year, can we programmatically use the value from https://github.com/python/devguide/blob/main/include/release-cycle.json?
(from #1582 (comment))

This is an attempt to do so generated in conjunction with openai's codex.

It introduces the first non-purely-declarative code into conf.py. Maybe there's a better place to put the code but I'm not familiar with the code base.

This screenshot shows that things work as expected (tested on commit 6da61e3)

https://cpython-devguide--1583.org.readthedocs.build/versions/

---

📚 Documentation preview 📚: https://cpython-devguide--1583.org.readthedocs.build/

[willingc]

This seems reasonable to me. Leaving it to our release expert @hugovk 👋 to review and merge. Thanks @corneliusroemer for working on improvements to the devguide too!

[hugovk]

Thanks! A couple of suggestions.

[corneliusroemer]

Thanks for the suggestions @hugovk - I have accepted them and added a small additional fix (renaming main_version -> _main_version in conf.py). I also tested the RTD test build output is as expected.

[hugovk]

Thank you!