keyword: use Python 2/3-agnostic set notation in reduce_keywords doctest

keyword: use Python 2/3-agnostic set notation in reduce_keywords doctest

Python 2.7 prints sets as "set([1, 2])", but Python 3.2 prints them as
"{1, 2}".  Avoid having to chose by showing that the result of
reduce_keywords() is a set, and then printing the elements in a list.