ReadTheDocs build (still) failing

[qstokkink]

It appears our .readthedocs.yaml is missing a build.os definition.

Error

The configuration key "build.os" is required to build your documentation. Read more at https://docs.readthedocs.io/en/stable/config-file/v2.html#build-os

[qstokkink]

The original error was fixed but the build still fails, at this step now:

python -m sphinx -T -E -b html -d _build/doctrees -D language=en . $READTHEDOCS_OUTPUT/html

WARNING: Invalid configuration value found: 'language = None'. Update your configuration to a valid language code. Falling back to 'en' (English).
making output directory... done
WARNING: html_static_path entry '_static' does not exist
WARNING: The pre-Sphinx 1.0 'intersphinx_mapping' format is deprecated and will be removed in Sphinx 8. Update to the current format as described in the documentation. Hint: "intersphinx_mapping = {'<name>': ('https://docs.python.org/', None)}".https://www.sphinx-doc.org/en/master/usage/extensions/intersphinx.html#confval-intersphinx_mapping

...

�[2Kwriting output... [  0%] autoapi/latest/ipv8/REST/asyncio_endpoint/index

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-ipv8/envs/latest/lib/python3.11/site-packages/sphinx/builders/html/__init__.py", line 1127, in handle_page
    output = self.templates.render(templatename, ctx)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-ipv8/envs/latest/lib/python3.11/site-packages/readthedocs_ext/readthedocs.py", line 181, in rtd_render
    content = old_render(template, render_context)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-ipv8/envs/latest/lib/python3.11/site-packages/sphinx/jinja2glue.py", line 200, in render
    return self.environment.get_template(template).render(context)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-ipv8/envs/latest/lib/python3.11/site-packages/jinja2/environment.py", line 1291, in render
    self.environment.handle_exception()
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-ipv8/envs/latest/lib/python3.11/site-packages/jinja2/environment.py", line 925, in handle_exception
    raise rewrite_traceback_stack(source=source)
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-ipv8/envs/latest/lib/python3.11/site-packages/sphinx/themes/basic/page.html", line 10, in top-level template code
    {%- extends "layout.html" %}
    ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-ipv8/envs/latest/lib/python3.11/site-packages/sphinx_rtd_theme/layout.html", line 67, in top-level template code
    <link rel="stylesheet" href="{{ pathto('_static/' + style, 1) }}" type="text/css" />
    ^^^^^^^^^^^^^^^^^^^^^^^^^
jinja2.exceptions.UndefinedError: 'style' is undefined

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-ipv8/envs/latest/lib/python3.11/site-packages/sphinx/cmd/build.py", line 298, in build_main
    app.build(args.force_all, args.filenames)
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-ipv8/envs/latest/lib/python3.11/site-packages/sphinx/application.py", line 355, in build
    self.builder.build_update()
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-ipv8/envs/latest/lib/python3.11/site-packages/sphinx/builders/__init__.py", line 293, in build_update
    self.build(to_build,
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-ipv8/envs/latest/lib/python3.11/site-packages/sphinx/builders/__init__.py", line 363, in build
    self.write(docnames, list(updated_docnames), method)
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-ipv8/envs/latest/lib/python3.11/site-packages/sphinx/builders/__init__.py", line 571, in write
    self._write_serial(sorted(docnames))
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-ipv8/envs/latest/lib/python3.11/site-packages/sphinx/builders/__init__.py", line 581, in _write_serial
    self.write_doc(docname, doctree)
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-ipv8/envs/latest/lib/python3.11/site-packages/sphinx/builders/html/__init__.py", line 655, in write_doc
    self.handle_page(docname, ctx, event_arg=doctree)
  File "/home/docs/checkouts/readthedocs.org/user_builds/py-ipv8/envs/latest/lib/python3.11/site-packages/sphinx/builders/html/__init__.py", line 1134, in handle_page
    raise ThemeError(__("An error happened in rendering the page %s.\nReason: %r") %
sphinx.errors.ThemeError: An error happened in rendering the page autoapi/latest/ipv8/REST/asyncio_endpoint/index.
Reason: UndefinedError("'style' is undefined")

Theme error:
An error happened in rendering the page autoapi/latest/ipv8/REST/asyncio_endpoint/index.
Reason: UndefinedError("'style' is undefined")