Test with multiple python versions.

.github/workflows/test-ubuntu.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python: [python, python3]
+        python-version: ['3.8.18', '3.9.21', '3.10.15', '3.11.11', '3.12.9', '3.13.2']
         cxx: [g++, clang++]
         std: [c++98, c++11, c++14, c++17]
         include:
@@ -27,10 +27,13 @@ jobs:
 
     steps:
     - uses: actions/checkout@v4
+    - uses: actions/setup-python@v5
+      with:
+        python-version: ${{ matrix.python-version }}
 
     - name: build
       run: |
-        ${{ matrix.python }} --version
+        python --version
         ${{ matrix.cxx }} --version
         faber -v
         sed -e "s/\$PYTHON/${{ matrix.python }}/g" .ci/faber > ~/.faber