input: Write a python function to tokenize a string, removing punctuation.
  assert tokenize('This sentence has commas, periods, and colons.') == ['This', 'sentence', 'has', 'commas', 'periods', 'and', 'colons']
  assert tokenize('Hello, world! How are you?') == ['Hello', 'world', 'How', 'are', 'you']
  assert tokenize('No punctuation here') == ['No', 'punctuation', 'here']
output: def tokenize(s):
      import re
      return re.findall(r'\b\w+\b', s)