Upgrade cache action

Author: logandk

.github/workflows/tests.yaml

@@ -31,7 +31,7 @@ jobs:
         id: pip-cache
         run: echo "::set-output name=dir::$(pip cache dir)"
       - name: cache pip
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           path: ${{ steps.pip-cache.outputs.dir }}
           key: pip|${{ runner.os }}|${{ matrix.python }}|${{ hashFiles('setup.py') }}|${{ hashFiles('requirements.txt') }}