Add keys for caching

.github/workflows/tests.yml

@@ -33,7 +33,13 @@ jobs:
       uses: actions/setup-python@v4
       with:
         python-version: ${{ matrix.config[0] }}
+        # for caching, see 
+        # https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#caching-packages
         cache: 'pip'
+        cache-dependency-path: |
+          setup.*
+          tox.ini
+          requirements*.txt
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip