input: Write a python function to split a string on punctuation characters.
  assert split_on_punctuation('Hello,World!')==['Hello', 'World!']
  assert split_on_punctuation('Python,is,great')==['Python', 'is', 'great']
  assert split_on_punctuation('This, is, a, test.')==['This', ' is', ' a', ' test.']
output: def split_on_punctuation(s):
      import re
      return re.split(r'([,!.])', s)