cis: use setup-python v2 for docs pipeline

.github/workflows/docs.yml

@@ -22,7 +22,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python
-        uses: actions/setup-python@v1
+        uses: actions/setup-python@v2
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip