.github/workflows/python-app.yml: use Python 3.10

That's what I use for testing locally. Note that we need to quote
the version here because otherwise GitHub Actions will interpret it
as 3.1 due to how YAML works.

.github/workflows/python-app.yml

@@ -16,10 +16,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python 3.9
+    - name: Set up Python 3.10
       uses: actions/setup-python@v2
       with:
-        python-version: 3.9
+        python-version: '3.10'
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip