change python version to 3.9

.github/workflows/pythonapp.yml

@@ -9,10 +9,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v1
-    - name: Set up Python 3.7
+    - name: Set up Python 3.9
       uses: actions/setup-python@v1
       with:
-        python-version: 3.7
+        python-version: 3.9
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip