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.

[bastimeyer]

myst-parser >=4 has dropped support for py38 and py39. Netlify is still stuck on py38 and there's no upgrade in sight. They simply don't care about Python. Being stuck on myst-parser >=1.0.0,<4 could introduce problems in the future though if they introduce breaking changes which affect our documentation sources. Then Streamlink packagers who build the docs and man pages can't use the latest myst-parser versions. I doubt that there will be any breaking changes which will affect us, but it's possible.

• https://github.com/streamlink/streamlink/blob/6.8.3/docs-requirements.txt#L10
• https://github.com/executablebooks/MyST-Parser/releases/tag/v4.0.0

Previews for GitHub-pages builds are still not supported, same as deploying to a different repo, so dropping Netlify in favor of GH actions for docs previews is currently impossible:

• When the preview feature will be available actions/deploy-pages#337
• Deploy to another repository actions/deploy-pages#308