CI: Add test matrix for validating different Sphinx versions

Author: amotl

.github/workflows/main.yml

@@ -23,13 +23,22 @@ jobs:
       fail-fast: false
       matrix:
         os: ["ubuntu-latest"]
-        python-version: ["3.9", "3.14"]
+        python-version: ["3.10", "3.14"]
+        sphinx-version: ["6.*", "7.*"]
+        include:
+          - os: "ubuntu-latest"
+            python-version: "3.9"
+            sphinx-version: "7.*"
 
     env:
       OS: ${{ matrix.os }}
       PYTHON: ${{ matrix.python-version }}
 
-    name: Python ${{ matrix.python-version }} on OS ${{ matrix.os }}
+    name: "
+    Sphinx ${{ matrix.sphinx-version }}
+    Python ${{ matrix.python-version }}
+    OS ${{ matrix.os }}
+    "
     steps:
 
     - name: Acquire sources
@@ -50,6 +59,9 @@ jobs:
         # https://github.com/pypa/setuptools/blob/main/CHANGES.rst#v6400
         pip install "setuptools>=64" --upgrade
 
+        # Install Sphinx beforehand, to implement test matrix.
+        pip install sphinx==${{ matrix.sphinx-version }}
+
         # Install package in editable mode.
         pip install --use-pep517 --prefer-binary --editable=.[develop,docs,test]