debian/pyhtml2devhelp.py generates a manual in devhelp format v1, which
was replaced by format v2 in 2005 and deprecated in 2018. Applications
based on the foundry library, like manuals (which replaced devhelp in
testing/unstable) and gnome-builder, no longer support loading devhelp
format v1 libraries.

One way to address this would be to make pyhtml2devhelp.py generate a
.devhelp2 file, instead of the current .devhelp.gz, and adapt its
contents to format v2. I attach some patches to do this.

Alternatively, the packaging could use python3-sphinxcontrib.devhelp,
which provides an extended version of sphinx's HTML output that adds a
devhelp index, and was recently enhanced to generate devhelp format v2.
This would avoid needing to screen-scrape Sphinx's HTML output, which
doesn't work particularly well because it's very sensitive to the exact
HTML that's generated for the index (I'll open a separate bug for the
incomplete index entries that result from this).

Or the index could perhaps be generated by parsing Sphinx's internal
data (searchindex.js or objects.inv, or some temporary file from which
they are generated), but I don't know enough about Sphinx to know which
route would be the best for doing that.

Thanks,
    smcv