Enable pip cache on GitHub Actions

.github/workflows/flask.yml

@@ -37,9 +37,10 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v1
+      uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
+        cache: 'pip'
 
     - name: Install dependencies
       run: |