Specify Python 3.8 on the workflow for a issue of pylint 2.6.0 on Python 3.9

pylint-dev/pylint#3882

Author: kmyk

.github/workflows/format.yml

@@ -11,6 +11,8 @@ jobs:
 
     - name: Set up Python
       uses: actions/setup-python@v1
+      with:
+        python-version: 3.8
 
     - name: Install dependencies
       run: pip3 install .[dev]