Update docs workflow

.github/workflows/sphinx.yml

@@ -10,7 +10,7 @@ jobs:
  - uses: actions/setup-python@v3
  - name: Install dependencies
  run: |
- pip install sphinx sphinx_rtd_theme furo sphinx_copybutton docutils sphinx_inline_tabs sphinx_removed_in
+ pip install sphinx sphinx_rtd_theme furo sphinx_copybutton docutils sphinx_inline_tabs sphinx_removed_in sphinxext
  - name: Sphinx build
  run: |
  sphinx-build -b html docs/source/ docs/build/html