str + bytes bug in save_html()

There is a [bug on the line 197](https://github.com/lightning-viz/lightning-python/blob/7ab5845e8cb1ff6b0dfa020f4a8ee0d95f4374bf/lightning/visualization.py#L197) in saving html method of `VisualizationLocal` class that results in

```
TypeError: Can't convert 'bytes' object to str implicitly
```

A possible fix:

```
f.write(('<script>' + js + '</script>').encode('utf-8'))
```
