Use Python 3.9 for linting

.github/workflows/main.yml

@@ -100,6 +100,8 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: jupyterlab/maintainer-tools/.github/actions/base-setup@v1
+        with:
+          python-version: '3.9'
       - name: Run Linters
         run: |
           hatch run typing:test