ModuleNotFoundError: No module named 'formgrade_extension'

Thanks a lot. I'd like to use the demo for getting nbgrader running on your own machine. However, there is a "ModuleNotFoundError" error, as I follow the steps of Docker file. 

```bash
pip install git+git://github.com/jupyter/nbgrader.git
jupyter serverextension enable --sys-prefix formgrade_extension
```