github: add venv caching

.github/workflows/tests.yaml

@@ -22,9 +22,15 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Set up Python
+        id: setup_python
         uses: actions/setup-python@v4
         with:
           python-version: '3.10'
+      - name: Cache virtual environment
+        uses: actions/cache@v3
+        with:
+          key: venv-${{ runner.os }}-${{ steps.setup_python.outputs.python-version}}
+          path: .venv
       - name: Install system packages
         run: sudo apt-get install -y portaudio19-dev
       - name: Install basic dependencies