Build: do not set sphinx_rtd_theme theme automatically

[humitos]

We are not installing sphinx-rtd-theme automatically on new projects. However, we are setting sphinx_rtd_theme automatically on their docs/conf.py.

This PR removes that magic to auto-set the theme and leave the theme to what the user has defined.

• Related: Stop creating a conf.py/mkdocs.yaml file automatically for users and doing magic  #2483
• Closes: Missing sphinx_rtd_theme required by default when appending docs/conf.py #10626

[ericholscher]

There's a bunch of stuff in the conf.py override it seems like we can remove (definitelly the "websupport2" stuff, and the source suffix), that won't effect much.

This actually will change the default theme for users without a theme set, but hopefully most users now are defining an html_theme, since Sphinx defaults to one defined, so this won't impact many users.

[humitos]

There's a bunch of stuff in the conf.py override it seems like we can remove (definitelly the "websupport2" stuff, and the source suffix), that won't effect much.

I opened a PR to remove websupport2 at #10646. We cannot remove the source_suffix for now because it's being used in our readthedocs-sphinx-ext at https://github.com/rtfd/readthedocs-sphinx-ext/blob/7c60d1646c12ac0b83d61abfbdd5bcd77d324124/readthedocs_ext/readthedocs.py#L155

However, I'm 👍🏼 on review all the context we are creating all around this and gradually migrate it to the new addons. That way, we will moving forward to the new implementation and figure it out what are the things we may be missing there.

[jpmckinney]

This is causing build errors

Running Sphinx v7.2.2

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/pelican-frontend/envs/latest/lib/python3.11/site-packages/sphinx/config.py", line 358, in eval_config_file
    exec(code, namespace)  # NoQA: S102
    ^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/pelican-frontend/checkouts/latest/docs/conf.py", line 138, in <module>
    'using_theme': using_rtd_theme,
                   ^^^^^^^^^^^^^^^
NameError: name 'using_rtd_theme' is not defined

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

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/pelican-frontend/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/pelican-frontend/envs/latest/lib/python3.11/site-packages/sphinx/application.py", line 211, in __init__
    self.config = Config.read(self.confdir, confoverrides or {}, self.tags)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/pelican-frontend/envs/latest/lib/python3.11/site-packages/sphinx/config.py", line 181, in read
    namespace = eval_config_file(filename, tags)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/pelican-frontend/envs/latest/lib/python3.11/site-packages/sphinx/config.py", line 371, in eval_config_file
    raise ConfigError(msg % traceback.format_exc()) from exc
sphinx.errors.ConfigError: There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/pelican-frontend/envs/latest/lib/python3.11/site-packages/sphinx/config.py", line 358, in eval_config_file
    exec(code, namespace)  # NoQA: S102
    ^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/pelican-frontend/checkouts/latest/docs/conf.py", line 138, in <module>
    'using_theme': using_rtd_theme,
                   ^^^^^^^^^^^^^^^
NameError: name 'using_rtd_theme' is not defined


Configuration error:
There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/pelican-frontend/envs/latest/lib/python3.11/site-packages/sphinx/config.py", line 358, in eval_config_file
    exec(code, namespace)  # NoQA: S102
    ^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/pelican-frontend/checkouts/latest/docs/conf.py", line 138, in <module>
    'using_theme': using_rtd_theme,
                   ^^^^^^^^^^^^^^^
NameError: name 'using_rtd_theme' is not defined

[humitos]

@jpmckinney Thanks. Yeah, we saw that and we just fixed it in #10653. We are doing a re-deploy now to solve this issue.

[bdarnell]

This actually will change the default theme for users without a theme set, but hopefully most users now are defining an html_theme, since Sphinx defaults to one defined, so this won't impact many users.

FWIW I somehow ended up with logic to set html_theme only if the build is not happening on RTD, so this broke my build. I can't remember why I did it that way but judging by the comment there was something weird about the way sphinx_rtd_theme was installed (or not) that prevented it from being imported.

https://github.com/tornadoweb/tornado/blob/a19263454ae62523fa586959b83bc0903c021697/docs/conf.py#L87-L96

[st-bender]

Hi,
Yeah, I was wondering if my builds were the only ones failing.
Please put at least a default theme back, to avoid having a lot of builds failing.