This change adds a new @api_stub decorator that we can use to define

stubs within resource implementations. These stubs provide type hints

and documentation for any methods which will be dynamically generated.
We have only a subset of subclasses for resources. For the ones we do

currently have, this adds stubs to the root resource for the methods

that are automatically generated from URI-templates. With this, if one's

editor is so configured, a developer can write

api_root.get_review_request( and the editor will auto-complete the

review_request_id parameter and indicate that the resulting value is a

ReviewRequestItemResource.

Ran unit tests.
Verified that stub methods were correctly replaced, and that doing

  things like help(api_root.get_review_request) showed the correct

  documentation and annotations.