[enh] CI: use actions/cache@v2 for Python dependencies

.github/workflows/integration.yml

@@ -22,7 +22,14 @@ jobs:
       with:
         python-version: ${{ matrix.python-version }}
         architecture: 'x64'
+    - name: Cache Python dependencies
+      id: cache-python
+      uses: actions/cache@v2
+      with:
+        path: ./local
+        key: python-${{ matrix.os }}-${{ matrix.python-version }}-${{ hashFiles('requirements*.txt', 'setup.py') }}
     - name: Install Python dependencies
+      if: steps.cache-python.outputs.cache-hit != 'true'
       run: |
         make V=1 install
         make V=1 gecko.driver