CI: Re-add missing actions/setup-python recipe

Otherwise, CI/GHA will (currently) always use Python 3.12,
which seems to be the default Python on the selected runner image.

Author: amotl

.github/workflows/tests.yml

@@ -53,6 +53,11 @@ jobs:
         with:
           persist-credentials: false
 
+      - name: Set up Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: ${{ matrix.python-version }}
+
       - name: Set up uv
         uses: astral-sh/setup-uv@v7
         with: