Use Python 3.6 to generate the API documentation

.github/workflows/publish_api_documentation.yml

@@ -12,6 +12,8 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - uses: actions/setup-python@v2
+      with:
+        python-version: 3.6
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip