Remove Sphinx<5 compatibility leftovers

[humitos]

• Remove blocks with logic for Sphinx<5
• Always use writer-html5 CSS class
• Use css_tag for better compatibility with newer Sphinx versions

• Closes Require Sphinx2 #1075
• Closes arbitrary add_css_file attributes are not supported #1100
• Closes layout: allow arbitrary CSS attributes #1101
• Closes Use .filename attribute for CSS and JS files for pathto #1503
• Closes Set data-content_root for Sphinx ≥ 7.2 #1507

@agjohnson this PR is a good candidate to be included in 2.0rc2 👍🏼 . If you approve it, I can try to make a release myself.

[humitos]

I tested this visually as well with Sphinx 5, 6 and 7 and I didn't find any issue so far.

[agjohnson]

The changes here all seem reasonable, however I'd probably say we should plan out release structure a bit more. At very least, it makes sense to focus 2.0rc1 on just changing dependencies, not changing code yet. Then at least projects can upgrade dependencies and transitive dependencies without us changing anything major about their project output.

So for that, I'd probably say a 2.0rc1 with dependencies upgraded and a 2.1rc1 following with actual changes to our theme. In that case, we'd block this for a bit longer.

If we need any of these changes specifically for Sphinx 7 support, they can be broken away from some of the cleanup changes here. But this looks mostly like housekeeping.

[humitos]

@agjohnson I split the required changes to support Sphinx >= 7.2 into #1519 and merge that one. I also update this PR with the latest changes.

Now, this PR is only and cleanup of the support for older (now) un-supported versions.

[agjohnson]

With 2.0 released, I think we're ready to merge this and prepare a 2.1a release. We haven't had any more feedback on the CSS changes in the 2.0rc{1,2,3,4} releases, so I think we're safe to continue on.

@humitos thoughts here?

[humitos]

Sounds good to me! I've updated the PR to resolve the conflicts.

[humitos]

This PR is ready to re-review and merge. I kept the injection of css_files code as it's on master which is working properly. On the other hand, I added a small comment for extra_css_files so we don't get confused again with that.