`UserAgent` raises a mypy error

A simple example:

```python
foo = UserAgent()
foo["device"]
```

Raises:

> Value of type "UserAgent" is not indexable  [index]

I think this has something to do with `attrs`, but haven't used it much in anger to understand the right way to fix it.

There may be a need to refactor to use `@attrs.define` instead of `@attr.s`, per https://www.attrs.org/en/stable/types.html