TST: Do not use Python 3.10 with linkcheck job

.github/workflows/ci_workflows.yml

@@ -105,7 +105,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v2
       with:
-        python-version: '3.x'
+        python-version: '3.9'
     - name: Install and build
       run: |
         python -m pip install --upgrade pip setuptools wheel sphinx-astropy