Add pip dependency cache

.github/workflows/ci.yml

@@ -11,12 +11,22 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 30
     steps:
+
     - uses: actions/checkout@v1
+
     - name: Set up Python 3.8
       uses: actions/setup-python@v1
       with:
         python-version: 3.8
 
+    - name: Cache pip deps
+      uses: actions/cache@v3
+      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