infra: pin the linter check to only use Python 3.9 (#766)

.github/workflows/check-format.yml

@@ -20,7 +20,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@65d7f2d534ac1bc67fcd62888c5f4f3d2cb2b236 # v4.7.1
       with:
-        python-version: '3.x'
+        python-version: '3.9'
     - name: Install dependencies
       run: |
         pip install --upgrade pip