Speed-up test workflow.

.github/workflows/test-package.yml

@@ -87,7 +87,20 @@ jobs:
       uses: actions/setup-python@v4
       with:
         python-version: ${{ matrix.python-version }}
-        cache: 'pip'
+        # cache: 'pip'
+
+    - name: Cache virtualenv
+      uses: actions/cache@v3
+      with:
+        key: venv-${{ runner.os }}-${{ matrix.python-version }}-${{ hashFiles('requirements.txt') }}-${{ hashFiles('requirements-test.txt') }}
+        path: ./venv
+
+    - name: Create virtualenv
+      run: |
+        python -m venv ./venv
+        source ./venv/bin/activate
+        echo "$VIRTUAL_ENV/bin" >> $GITHUB_PATH
+        echo "VIRTUAL_ENV=$VIRTUAL_ENV" >> $GITHUB_ENV
 
     - name: Upgrade pip version
       run: |