cpython#121277: Replace next versions in docs by the just-released version

[encukou]

See CPython issue: python/cpython#121277

After the CPython PR is merged, a Sphinx plugin will expand next in directives like versionchanged to e.g. 3.14.0b0 (unreleased).

This PR gives the RM a tool to replace all such occurences of next by the currently released version, baking it directly into the ReST source files, as part of the version bump commit.

This is done textually (I don't know of a round-trip parser for ReST).

To make sure the text manipulation worked, the released docs artifact (if it's being published) is grepped for the string (unreleased), and if it's found, the RM is asked for confirmation.

[encukou]

Dear RMs, do you want this?

I can add tests for the remaining uncovered lines; is this the style of tests you're looking for?

[hugovk]

Dear RMs, do you want this?

Yes, I think so, but still need to take a closer look, and would also like to hear from other RMs who have actually made releases :)

I can add tests for the remaining uncovered lines; is this the style of tests you're looking for?

I've not had a proper look yet, but will do.

[hugovk]

Looking good, I tested it against python/cpython#121278 and it worked as expected.

Here's a few small suggestions.

[hugovk]

Thanks!

[encukou]

On the sprint, @Yhg1s agreed to the concept, so I'll merge.

I'll link related PRs on the CPython issue: python/cpython#121277