Fix deprecated action

Author: lucagiove

.github/workflows/pythonapp.yml

@@ -24,7 +24,7 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python
-      uses: actions/setup-python@v1.2.0
+      uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
     - name: Display Python version