Pin github workflow python version to fix install issue (old dependency)

.github/workflows/main.yml

@@ -16,6 +16,8 @@ jobs:
           submodules: true
       - name: setup Python
         uses: actions/setup-python@v2
+        with:
+          python-version: '3.7'
       - name: install Poetry
         uses: snok/install-poetry@v1.2.1
       - name: set poetry config path