Merge pull request sphinx-doc#9768 from tk0miya/fix_transifex_client

Fix CI: Downgrade python for transifex CI

Author: tk0miya

.github/workflows/transifex.yml

@@ -35,6 +35,8 @@ jobs:
         ref: 4.x
     - name: Set up Python
       uses: actions/setup-python@v2
+      with:
+        python-version: 3.9  # https://github.com/transifex/transifex-client/pull/330
     - name: Install dependencies
       run:  pip install -U babel jinja2 transifex-client
     - name: Extract translations from source code