.github/workflows/pages.yml: quote Python version

Otherwise, in JSON, it gets interpreted as an integer (3.1).

See: https://github.com/actions/setup-python/issues/160

.github/workflows/pages.yml

@@ -15,7 +15,7 @@ jobs:
           # 2022-01-06 this is currently an OK choice, but it will need to be
           # updated eventually as the ecosystem moves. Keep in sync with the
           # version in pyproject.toml.
-          python-version: 3.10
+          python-version: '3.10'
       - name: Install Poetry
         uses: abatilo/actions-poetry@v2.1.4
       - name: Install Python dependencies