build: use python 3.6 for linting

since Python 3.5 reached the end of its life on September 13th, 2020.

Author: weakish

.github/workflows/pythonpackage.yml

@@ -25,7 +25,7 @@ jobs:
     - name: Set up Python 
       uses: actions/setup-python@v2
       with:
-        python-version: '3.5'
+        python-version: '3.6'
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip