add caching for pip packages

.github/workflows/python-tests.yml

@@ -26,6 +26,8 @@ jobs:
       uses: actions/setup-python@v6
       with:
         python-version: ${{ matrix.python-version }}
+        cache: 'pip'
+        cache-dependency-path: pyproject.toml
     - name: Install FFmpeg & Poppler 
       run: |
         sudo apt update