Lock python version

Author: koba-e964

.github/workflows/format.yml

@@ -12,7 +12,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v4
       with:
-        python-version: 3.x
+        python-version: '3.8'
 
     - name: Install dependencies
       run: pip3 install .[dev]