Fun post!
I believe you intended to run in python3 based on your last cell.
changing maketrans and translate will have it run in python2.7
table = string.maketrans('', '') # remove punctuation from each word
stripped = [w.translate(table, string.punctuation) for w in tokens]
I did not read the book on nltk. I've been into information/coding theory recently (just finished Information by James Gleick) and thought I'd try my hand at something closer to NLP/ML. Very little background in the topic - I've taken a few courses in college and chatted with a few friends that know ML fairly well, but besides that I relied on blog posts and papers found on arxiv!