Update Sphinx version

[StanFromIreland]

We are running v3.4.3 causing failures as it uses deprecated features.

[StanFromIreland]

Error now gone, I presume it was fixed by Sphinx team?

[m-aciek]

It looks like setup-python GitHub Action is transitioning to 3.13 from 3.12. Python 3.13 removed imghdr standard library module, so it's causing issues. Sphinx 6.2+ doesn't depend on imghdr, but 3.10 and 3.9 branches (at least) pin Sphinx to 3.4.3 afaic.

Let's check if this will affect the CPython docs builds for older branches. If so, we need to fix it upstream.

[StanFromIreland]

Maybe you should post a message in the docs discord.

[m-aciek]

For the record, CPython and we pinned the Python 3.12 for the Python 3.10 version.