Pin qutip version to 4 to let the test pass

.github/workflows/test.yml

@@ -104,6 +104,7 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         python -mpip install -r doc/requirements.txt
+        pip install qutip==4.7.3
         pip install .
     - name: Test code snippets in the documentation
       run: |