Good question. I considered using sets but then decided to use lists because they don't get rid of duplicates and would make the test fail in the (indeed unlikely) case that a keyword gets added twice (that's actually not possible in Test.keywords because it's a dict, but keyword.kwlist is a list and the implementation of either one could change at some point).
In any case it probably doesn't make much difference -- I just preferred to err on the safe side.