Actually test with matrix sphinx version (#11)

Drop Sphinx 2-4, broken on newer pythons

.github/workflows/CI.yml

@@ -36,7 +36,7 @@ jobs:
     strategy:
       matrix:
         python-version: ['3.8', '3.9', '3.10', '3.11', '3.12', 'pypy3.9']
-        sphinx-version: ['2', '3', '4', '5', '6', '7']
+        sphinx-version: ['5.*', '6.*', '7.*']
 
     name: "Test - Python(${{ matrix.python-version }}), Sphinx(${{ matrix.sphinx-version }})"
     steps:
@@ -50,6 +50,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install -r ./dev-requirements.txt
+          python -m pip install sphinx==${{ matrix.sphinx-version }}
 
       - name: Download build artifacts
         uses: actions/download-artifact@v4