Upgrade to Furo 2023.9.10 to fix docs build

[hugovk]

Description

I spotted that https://black.readthedocs.io/en/stable/ is still pointing to 23.7.0 and https://black.readthedocs.io/en/latest/ to 23.7.1.dev33, instead of both at 2.3.9.0[.devXX].

The last Read the Docs build failed with:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/black/envs/latest/lib/python3.11/site-packages/sphinx/cmd/build.py", line 293, in build_main
    app = Sphinx(args.sourcedir, args.confdir, args.outputdir,
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/black/envs/latest/lib/python3.11/site-packages/sphinx/application.py", line 272, in __init__
    self._init_builder()
  File "/home/docs/checkouts/readthedocs.org/user_builds/black/envs/latest/lib/python3.11/site-packages/sphinx/application.py", line 343, in _init_builder
    self.events.emit('builder-inited')
  File "/home/docs/checkouts/readthedocs.org/user_builds/black/envs/latest/lib/python3.11/site-packages/sphinx/events.py", line 96, in emit
    results.append(listener.handler(self.app, *args))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/black/envs/latest/lib/python3.11/site-packages/furo/__init__.py", line 234, in _builder_inited
    raise ConfigError(
sphinx.errors.ConfigError: Furo is being used as an extension in a non-HTML build. This should not happen.

Configuration error:
Furo is being used as an extension in a non-HTML build. This should not happen.

This looks a Furo bug, and was fixed yesterday in pradyunsg/furo@5abeb9f, but has not been released yet (cc @pradyunsg).

I checked latest Furo main (furo @ git+https://github.com/pradyunsg/furo@main in requirements.txt) and it built fine.

Whilst awaiting a new release, let's use the last working version.

---

I also recommend enabling build previews for PRs at https://readthedocs.org/dashboard/black/advanced/ so future RTD build failures don't slip by.

Docs: https://docs.readthedocs.io/en/stable/pull-requests.html

Checklist - did you ...

• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

[pradyunsg]

You have uncanny timing @hugovk! https://pypi.org/project/furo/2023.9.10/ :)

[hugovk]

Thanks! Let me flip this to an upgrade instead :)

[JelleZijlstra]

Thanks Hugo for the PR and Pradyun for the timely release!

[JelleZijlstra]

Also enabled PR builds in RTD as you recommended.