Always use Python 3.8 for tox in GitHub Actions

Author: domdfcoding

.github/workflows/python_ci_linux.yml

@@ -60,7 +60,9 @@ jobs:
         if: ${{ steps.changes.outputs.code == 'true' || steps.changes.outcome == 'skipped' }}
         uses: "actions/setup-python@v4"
         with:
-          python-version: "${{ matrix.config.python-version }}"
+          python-version: |
+            ${{ matrix.config.python-version }}
+            3.8
 
       - name: Install dependencies 🔧
         if: steps.setup-python.outcome == 'success'