Infra: bump Sphinx to >8.1, limit docutils to <0.22 by Secrus · Pull Request #4926 · python/peps

Let's not set a higher version than needed, but if we have a good reason for bumping it, I think that's fine.

Something to note, Sphinx does drop older Python sooner than many projects. For example, Sphinx 9.1.0 requires Python 3.12 or newer.

But then people writing PEPs are targeting changes in 3.15 and soon 3.16 :)