sage 9.0 docs fails to compile on Slackware

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I am packaging up SageMath for Slackware (via SlackBuilds), but I am running into a compilation error. I've tried this in two different ways - first just by a straight compile from source, and then I also tried running the SlackBuild script from Sage 8.0 (which works fine) and changing the version from 8 to 9.

(that is certainly not guaranteed to work, but I thought maybe this was a known problem in Slackware, and the script from the 8.0 version might have a fix - it did not).

The problem is the following:

[dochtml] Error building the documentation.
[dochtml] Traceback (most recent call last):
[dochtml]   File "/tmp/SBo/sage-9.0/local/lib/python3.7/runpy.py", line 193, in _run_module_as_main
[dochtml]     "__main__", mod_spec)
[dochtml]   File "/tmp/SBo/sage-9.0/local/lib/python3.7/runpy.py", line 85, in _run_code
[dochtml]     exec(code, run_globals)
[dochtml]   File "/tmp/SBo/sage-9.0/local/lib/python3.7/site-packages/sage_setup/docbuild/__main__.py", line 2, in <module>
[dochtml]     main()
[dochtml]   File "/tmp/SBo/sage-9.0/local/lib/python3.7/site-packages/sage_setup/docbuild/__init__.py", line 1720, in main
[dochtml]     builder()
[dochtml]   File "/tmp/SBo/sage-9.0/local/lib/python3.7/site-packages/sage_setup/docbuild/__init__.py", line 327, in _wrapper
[dochtml]     getattr(get_builder(document), 'inventory')(*args, **kwds)
[dochtml]   File "/tmp/SBo/sage-9.0/local/lib/python3.7/site-packages/sage_setup/docbuild/__init__.py", line 552, in _wrapper
[dochtml]     self._build_everything_except_bibliography(lang, format, *args, **kwds)
[dochtml]   File "/tmp/SBo/sage-9.0/local/lib/python3.7/site-packages/sage_setup/docbuild/__init__.py", line 538, in _build_everything_except_bibliography
[dochtml]     build_many(build_ref_doc, non_references)
[dochtml]   File "/tmp/SBo/sage-9.0/local/lib/python3.7/site-packages/sage_setup/docbuild/__init__.py", line 280, in build_many
[dochtml]     _build_many(target, args, processes=NUM_THREADS)
[dochtml]   File "/tmp/SBo/sage-9.0/local/lib/python3.7/site-packages/sage_setup/docbuild/utils.py", line 283, in build_many
[dochtml]     raise worker_exc.original_exception
[dochtml] OSError: /tmp/SBo/sage-9.0/src/doc/en/reference/algebras/sage/combinat/posets/moebius_algebra.rst:5: WARNING: undecodable source characters, replacing with "?": b'M>>>\xf6<<<bius Algebras'
Makefile:2019: recipe for target 'doc-html' failed
make[3]: *** [doc-html] Error 1

So it seems there is a strange character in the doc files for the algebras packages. I can imagine a few work-arounds - is there some way the OS can be told how to handle it? I could probably figure out how to build it without the documentation, but I would rather provide the documentation with the sources.