fix(ci): failing readthedocs build #553

ref #552

    python -m sphinx -T -E -W --keep-going -b html -d _build/doctrees -D language=en . $READTHEDOCS_OUTPUT/html
    Running Sphinx v7.2.6
    making output directory... done

    Traceback (most recent call last):
      File "/home/docs/checkouts/readthedocs.org/user_builds/pynvim/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/pynvim/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/pynvim/envs/latest/lib/python3.11/site-packages/sphinx/application.py", line 342, in _init_builder
        self.builder.init()
      File "/home/docs/checkouts/readthedocs.org/user_builds/pynvim/envs/latest/lib/python3.11/site-packages/sphinx/builders/html/__init__.py", line 219, in init
        self.init_templates()
      File "/home/docs/checkouts/readthedocs.org/user_builds/pynvim/envs/latest/lib/python3.11/site-packages/sphinx/builders/html/__init__.py", line 270, in init_templates
        self.theme = theme_factory.create(themename)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/home/docs/checkouts/readthedocs.org/user_builds/pynvim/envs/latest/lib/python3.11/site-packages/sphinx/theming.py", line 230, in create
        raise ThemeError(__('no theme named %r found (missing theme.conf?)') % name)
    sphinx.errors.ThemeError: no theme named 'sphinx_rtd_theme' found (missing theme.conf?)

    Theme error:
    no theme named 'sphinx_rtd_theme' found (missing theme.conf?)

.readthedocs.yml

@@ -17,6 +17,8 @@ sphinx:
   fail_on_warning: true
 
 python:
-   install:
-   - method: pip
-     path: .
+  install:
+  - method: pip
+    path: .
+    extra_requirements:
+      - sphinx_rtd_theme