Upgrade our version of sphinx so we get docutils 0.19

[TibsAtWork]

Our current requirements.txt specifies sphinx 4.3.2, which implicitly pins docutils to 0.17.1 (for compatibility issues of the time)

Docutils stable is now 0.19 - note this requires Python 3.7 or greater

Sphinx stable is at 5.3.0 (released Oct 16, 2022), and Sphinx 5.10 (from July) supports docutils 0.19

(note that Sphinx 6, which is still in testing, will drop support for Python 3.7 - our new downstream platform looks like it will only support 3.7, see #1528)

Vale uses Python and docutils / rst2html[.py] to process reStructuredText before it lints it. I'm a bit worried about using an out-of-date version of docutils for this purpose. It can also cause some confusion when I'm running tests on my machine, if my "general" docutils installation (0.19) is different than the one in my devportal virtual environment (0.17.1) - not helpful when I'm trying to debug things.

The complication would be checking that all of our other requirements continue to work with the new version of sphinx.

[TibsAtWork]

cloudflare/pages-build-image#1 may be a relevant discussion - it's been going on a while with no obvious progress, but I don't know how "official" it is. I've selected to follow any changes.

[dogukancagatay]

Cloudflare has done some modernization in their Cloudflare Pages build toolbox. Now, with the V2 build system, it is possible to have the Python version configured.

https://developers.cloudflare.com/pages/platform/language-support-and-tools/