This brings some long-needed support for working with modern

extra_data functionality in Review Board.
Attempting to modify an extra_data dictionary now raises an exception.

This is part of the work done to make dictionary fields immutable, since

RBTools doesn't have a concept of simply saving a resource back to the

server (update() calls are required instead). The immutable error

provides handy instructions for setting new extra_data content.
The big change here, though, is the support for extra_data patching.

This was doable before through something like:

resource.update(**{'extra_data:json': json.dumps({...})})


That obviously was not ideal. Now, callers can simply set the

extra_data_json field to a native dictionary to do a JSON Merge Patch

or set extra_data_json_patch to a list of dictionaries to do a JSON

Patch. RBTools will properly serialize the results to JSON, setting the

correct key in the payload.
A .copy() method also exists to return a copy of the original field

data, without any wrapping. This works for any dictionary level of

extra_data, allowing consumers to create a copy, manipulate it, and

set it as a new full dictionary in a patch request.
Between new patching options, a more standard dictionary interface, the

ability to copy the payload, and useful immutable error handling, it

should now be a lot easier to work with extra_data fields.

Unit tests pass on all supported versions of Python.