Typing error when using callback as body

[davetapley]

Per the docs passing a callback as body with register_uri causes:

(function) exceptionCallback: (request: Unknown, uri: Unknown, headers: Unknown) -> NoReturn
Argument of type "(request: Unknown, uri: Unknown, headers: Unknown) -> NoReturn" cannot be assigned to parameter "body" of type "str" in function "register_uri"
  "object" is incompatible with "str"```

I see a little bit of typing has made its way in, so am I good to make a PR adding types here?