UserAgent raises a mypy error

[miketheman]

A simple example:

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