ci: docs preview builds on netlify currently limited to sphinx 7.1.2

[bastimeyer]

Netlify is still stuck on Python 3.8:

• https://docs.netlify.com/configure-builds/available-software-at-build-time/
• https://answers.netlify.com/t/python-3-11-or-newer/84566

Since Sphinx 7.2.0 has already dropped support for Python 3.8, this means that all docs preview builds on netlify are built with Sphinx 7.1.2 instead of the latest one.

• https://www.sphinx-doc.org/en/master/changes.html#release-7-2-0-released-aug-17-2023
• https://github.com/streamlink/streamlink/blob/4f65b9a97d80b7ba45e6175d4a09d4c707035238/docs-requirements.txt

The docs dependencies must not be bumped when bumping would drop support for Python 3.8, or we would break the preview builds.