Upgrading cache to v4

Author: llavarello

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
       - uses: actions/setup-python@v4
         with:
           python-version: 3.x
-      - uses: actions/cache@v2
+      - uses: actions/cache@v4
         with:
           key: ${{ github.ref }}
           path: .cache