Update CI to test all Python and Sphinx combinations

Match CI test matrix to tox configuration:
- Python versions: 3.11, 3.12, 3.13
- Sphinx versions: 6, 7, 8
- Total: 9 test combinations

This ensures CI tests the same environments as local tox testing,
providing better coverage and catching version-specific issues.

Author: mmcky

.github/workflows/ci.yml

@@ -23,7 +23,8 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.11", "3.12"]
+        python-version: ["3.11", "3.12", "3.13"]
+        sphinx-version: ["6", "7", "8"]
     steps:
       - uses: actions/checkout@v4
       - name: Set up Python ${{ matrix.python-version }}
@@ -34,16 +35,17 @@ jobs:
         run : |
           python -m pip install --upgrade pip
           pip install -e.[testing]
+          pip install "sphinx>=${{ matrix.sphinx-version }},<${{ matrix.sphinx-version == '6' && '7' || matrix.sphinx-version == '7' && '8' || '9' }}"
       - name: Run pytest
         run: |
           pytest --durations=10 --cov=sphinx_exercise --cov-report=xml --cov-report=term-missing
       - name: Create cov
         run: coverage xml
       - name: Upload to Codecov
-        if: false && (matrix.python-version == '3.11')
+        if: false && (matrix.python-version == '3.11' && matrix.sphinx-version == '8')
         uses: codecov/codecov-action@v4
         with:
-          name: sphinx-exercise-pytest-py3.11
+          name: sphinx-exercise-pytest-py3.11-sphinx8
           token: "${{ secrets.CODECOV_TOKEN }}"
           flags: pytests
           file: ./coverage.xml