Deploy docs on publish using Python 3.9

A workaround for gcloud setup, see:

https://til.simonwillison.net/googlecloud/gcloud-error-workaround

Refs #1963

.github/workflows/publish.yml

@@ -69,7 +69,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v2
       with:
-        python-version: '3.10'
+        python-version: '3.9'
     - uses: actions/cache@v2
       name: Configure pip caching
       with: