URLMapTransport now supports the ?expand= query string argument,

allowing links to be expanded. When set, the transport will look up any

links listed and try to expand them in the resource before returning it

(and will also attempt to recurse into the expanded resources and expand

links within those).
If an expanded URL is not registered, or it's mapped to a HTTP 404

response, then the expansion will be empty (None or []).
Normally, a list will be expanded to a list, and a dictionary to a

dictionary. This can be overridden to expand a dictionary into a 1-item

list of dictionaries (needed for review request drafts, due to a design

flaw in the original API). This is done through a link_expansion_types

dictionary that can be set during URL registration.

Made use of this with some in-development unit tests. All tests pass on

all supported versions of Python.
Same for all existing tests as of this change.