Session04 - trigrams.py finished

Nice use of iteration to create construct the sets. I'm not sure that there's anything wrong with your comparisons (though there is a tingling in the back of my mind that suggests it's poor form to compare the value of `ints` and `floats` because it doesn't always workout the way you expect--perhaps @cewing will confirm or correct), but the conventional way to determine if a number is divisible by another is to use the modulo operator `%` and a comparison with zero. 

    if i % 2 == 0:
        s2.add(i)