This decorator allows us to move functions over to keyword-only

arguments without breaking callers that may be passing arguments as

position-only. It takes care of converting calls (using keyword

arguments or positional arguments) to the right format expected by the

function, emitting a deprecation warning if any keyword-only arguments

are passed as positional arguments.
This is a copy of the same method used in RBTools.
The deprecation warning classes have also been given a version

attribute, to help with generating suitable error messages.

Made use of this extensively with some in-progress work, with functions

using keyword-only arguments and taking *args and **kwargs, passing

them down to other methods using the decorator.