force python 3.7

.github/workflows/docs.yaml

@@ -23,6 +23,11 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+      - name: Set up Python
+        uses: actions/setup-python@v1
+        with:
+          python-version: '3.7'
+
       - name: Install Sphinx
         run: |
           pip install sphinx sphinx-rtd-theme