Update cache action to v4

Author: fabaff

.github/workflows/main.yml

@@ -27,7 +27,7 @@ jobs:
       - uses: snok/install-poetry@v1
       - name: cache deps
         id: cache-deps
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           path: .venv
           key: pydeps-${{ hashFiles('**/poetry.lock') }}