Caches pip in GH actions

Author: boriel

.github/workflows/python-app.yml

@@ -20,6 +20,13 @@ jobs:
       uses: actions/setup-python@v2
       with:
         python-version: 3.8
+    - name: Caches pip
+      uses: actions/cache@v1
+      with:
+        path: ~/.cache/pip
+        key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}
+        restore-keys: |
+          ${{ runner.os }}-pip-
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip