gh-117928: Bump the minimum Sphinx version to 6.2.1

[Eclips4]

Fixes https://github.com/python/cpython/security/dependabot/6

FYI, I've followed Victor's instructions from this comment.

• Issue: Increase minimum supported Sphinx to 6.2.1 #117928

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📚 Documentation preview 📚: https://cpython-previews--117853.org.readthedocs.build/

[nineteendo]

Sphinx version error:
The sphinxcontrib.applehelp extension used by this project needs at least Sphinx v5.0; it therefore cannot be built with this version.

[Eclips4]

Sphinx version error:
The sphinxcontrib.applehelp extension used by this project needs at least Sphinx v5.0; it therefore cannot be built with this version.

Thanks for pointing this out.
@hugovk @AA-Turner Maybe it's time to switch to Sphinx 5.0?

[hugovk]

Now that the 3.11 branch is only accepting security fixes, for 3.12+ I think we can bump the oldest supported Sphinx to at least 6.2.1.

Let's open a new issue to do a similar survey to #109209.

[hugovk]

Survey: #117928

[hugovk]

OK, according to #117928 we can upgrade to Sphinx 6.2.1 as the minimum for the 3.12 and main branches.

Would you like to update this PR? We can also use that issue number for this PR.

There's some compatibility code for old Sphinx we can remove from some files Doc/tools, and let's also rename master_doc in conf.py to root_doc (ref).

[Eclips4]

OK, according to #117928 we can upgrade to Sphinx 6.2.1 as the minimum for the 3.12 and main branches.

Would you like to update this PR? We can also use that issue number for this PR.

There's some compatibility code for old Sphinx we can remove from some files Doc/tools, and let's also rename master_doc in conf.py to root_doc (ref).

I'll update this PR as soon as possible ;)

[hugovk]

Thanks, let's also try using 3.12 in .github/workflows/reusable-docs.yml and put 6.2.1 in the comment:

        python-version: '3.11'  # known to work with Sphinx 4.2

[hugovk]

Thanks!

[miss-islington-app]

Thanks @Eclips4 for the PR, and @hugovk for merging it 🌮🎉.. I'm working now to backport this PR to: 3.12.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @Eclips4 and @hugovk, I could not cleanly backport this to 3.12 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 463c20dae9ac91d6c44028177b90f43238b819cf 3.12

[hugovk]

@Eclips4 Please could you backport?

[Eclips4]

@Eclips4 Please could you backport?

Will do it today!

[bedevere-app]

GH-118321 is a backport of this pull request to the 3.12 branch.