Docs: upgrade to python-docs-theme 2023.7

[hugovk]

The venv used on the docs server persists between deploys, therefore let's pin the theme so we get the version we want:

• https://github.com/python/python-docs-theme/releases/tag/2023.7

This means we'll get the dark theme for 3.11 (aka the main https://docs.python.org/3/). We've already had it on https://docs.python.org/3.12/ and https://docs.python.org/3.13/ for a few months.

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Let's also bump Sphinx to include the latest 6.x patch release:

• https://www.sphinx-doc.org/en/master/changes.html#release-6-2-1-released-apr-25-2023

(We have another PR to upgrade Sphinx to 7.x, let's handle that separately: #99381)

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📚 Documentation preview 📚: https://cpython-previews--107617.org.readthedocs.build/