.github: update actions

Switch to newer checkout and setup-python actions, and enable the
pip cache for setup-python.

.github/workflows/python-app.yml

@@ -27,11 +27,12 @@ jobs:
           - 5432:5432
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Set up Python 3.10
-      uses: actions/setup-python@v2
+      uses: actions/setup-python@v4
       with:
         python-version: '3.10'
+        cache: 'pip'
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip