Docs EPUB: fix meta tags connected with Open Graph

[m-aciek]

• Move meta theme-color tag from sphinxext-opengraph custom tags config directly to layout.html. Previously it would be added for all HTML-related Sphinx builders, after it's only added when builder == HTML.
• Partially revert Custom tags and EPUB builder sphinx-doc/sphinxext-opengraph#136 to discuss alternative fixes.)

It's a follow-up to #133720. This change should fix EPUB builds. When testing locally for #133720 I didn't have sphinxext-opengraph installed in the environment, that's why I didn't catch it earlier, I'm sorry.

Could we please backport it for supported versions and 3.12?

Before:

After:

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📚 Documentation preview 📚: https://cpython-previews--134071.org.readthedocs.build/

[hugovk]

Do meta tags even make sense for EPUB (and other builders)? If not, it's probably better to skip them altogether for those builders. Could save us future headaches as well.

[m-aciek]

Do meta tags even make sense for EPUB (and other builders)? If not, it's probably better to skip them altogether for those builders. Could save us future headaches as well.

It looks like they don't. https://chatgpt.com/share/6827445c-70f0-8004-bed3-353546e96afa

@methane started the implementation of skipping them on the extension side.

[hugovk]

It looks like they don't. chatgpt.com/share/6827445c-70f0-8004-bed3-353546e96afa

I'd prefer an authoritative source rather than a webpage that says "ChatGPT can make mistakes. Check important info."

[m-aciek]

It looks like they don't. chatgpt.com/share/6827445c-70f0-8004-bed3-353546e96afa

I'd prefer an authoritative source rather than a webpage that says "ChatGPT can make mistakes. Check important info."

I wasn't able to confirm that searching manually in a limited time.

[cclauss]

The epub download is still broken.

[hugovk]

Can we add a conditional in conf.py to skip these tags for non-HTML, or does it need to go in the sphinxext-opengraph extension or somewhere else?

[m-aciek]

Can we add a conditional in conf.py to skip these tags for non-HTML, or does it need to go in the sphinxext-opengraph extension or somewhere else?

In my opinion the cleanest would be to have it in sphinxext-opengraph, if/when sphinx-doc/sphinxext-opengraph#137 is merged. (Though the CI is failing there, probably it needs addressing first.)

Probably we could leverage tags to put this logic in conf.py.

[cclauss]

Can we please do as suggested at #134071 (comment) above?

It is painful not to be able to read the Python EPUB docs.

[hugovk]

@cclauss Would you like to open a PR?

[cclauss]

I would have opened a PR a long time ago if I knew how to fix this problem.