Attempt to install python3.6 in github workflow

Author: t-persson

.github/workflows/main.yml

@@ -20,7 +20,7 @@ jobs:
       - name: Setup python
         uses: actions/setup-python@v2
         with:
-          python-version: 3.6
+          python-version: 3.6.15
       - name: Install Tox
         run: pip install tox
       - name: Upgrade setuptools