Consider adding `ipywidgets` as a dependency.
When I install `datasets` in a fresh virtualenv with jupyterlab I always see this error. 

```
ImportError: IProgress not found. Please update jupyter and ipywidgets. See https://ipywidgets.readthedocs.io/en/stable/user_install.html
```

It's a bit of a nuisance, because I need to run shut down the jupyterlab server in order to install the required dependency. Might it be an option to just include it as a dependency here? 