update syntax

Author: parthea

.github/workflows/tests.yaml

@@ -42,7 +42,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
-          python-version: ${{ vars.DOCS_PYTHON }}
+          python-version: "${{ vars.DOCS_PYTHON }}"
           cache: 'pip'
       - name: Install nox.
         run: python -m pip install nox