Update the holoviz notebook to use pystac

The holoviz notebook example https://github.com/intake/intake-stac/blob/7706db898fff61375a75ea174a5de879893766e7/examples/intake-holoviz.ipynb is failing with

```
Expected instance, got:
```

I guess the notebook content has not been changed to reflect the migration from `statstac` to `pystac` https://github.com/intake/intake-stac/pull/72

I have quickly tried to change the imports and initialise a pystac catalog, but sounded like a less trivial task than expected looking at the doc https://pystac.readthedocs.io/en/stable/tutorials/how-to-create-stac-catalogs.html. I may take that back on my own, any snippet appreciated.