Update for Twitch API

Hey @PetterKraabol 

kudos for maintaining this, you're doing God's work.
Do you have plans to update the library to be compatible with the GraphQL API so it can get chats from VODs?

Due to the old `Helix` API being deprecated, this feature currently doesn't work:

```python
for comment in helix.video(318017128).comments:
    print(comment.commenter.display_name)
```

Cheers.