.github/workflows/python-app.yml: Use Python 3.9

I now use this version in my development environment. Eventually I
should add a matrix of versions to use, but I don't know the GitHub
Actions syntax well enough yet.

.github/workflows/python-app.yml

@@ -16,10 +16,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python 3.8
+    - name: Set up Python 3.9
       uses: actions/setup-python@v2
       with:
-        python-version: 3.8
+        python-version: 3.9
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip