Add sphinx-testing to CI dependencies

Updated the CI workflow to install the sphinx-testing package before running tests. This ensures that all required testing dependencies are available during CI runs.

Author: douden

.github/workflows/ci.yml

@@ -36,6 +36,7 @@ jobs:
         run : |
           python -m pip install --upgrade pip
           pip install -e.[testing]
+          pip install sphinx-testing
       - name: Run pytest
         run: |
           pytest --durations=10 --cov=sphinx_proof --cov-report=xml --cov-report=term-missing