ci: update the cache action to v4 (#11)

.github/workflows/tests.yml

@@ -42,7 +42,7 @@ jobs:
       run: poetry config virtualenvs.in-project true
 
     - name: Set up cache
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       id: cache
       with:
         path: .venv