terminable_thread provides a subclass of threading.Thread, adding
the facility to raise exceptions in the context of the given
thread.

This facility is incorporated in the terminable_thread.Thread
methods raise_exc, which raises an arbitrary exception, and
terminate, which raises SystemExit.

This is not done in an entirely robust manner, and there may be
unreported issues with it.

It uses the unexposed PyThreadState_SetAsyncExc function (via
ctypes) to raise an exception for the given thread.