input: Write a python function to calculate the similarity percentage between two strings.
  assert string_similarity('apple', 'appel') == 100.0
  assert string_similarity('abc', 'acb') == 66.66666666666666
  assert string_similarity('hello', 'billion') == 20.0
output: def string_similarity(str1, str2):
      from difflib import SequenceMatcher
      ratio = SequenceMatcher(None, str1, str2).ratio()
      return ratio * 100