Add cache

.github/workflows/ci.yaml

@@ -22,6 +22,10 @@ jobs:
 
     steps:
       - uses: actions/checkout@v2
+      - uses: actions/cache@v2
+        with:
+          key: ${{ github.ref }}
+          path: .cache
       - name: Set up python ${{ matrix.python-version }}
         uses: actions/setup-python@v2
         with: