input: Write a python function to find the top five most frequent characters in a given string.
  assert top_five_characters('aaabbbcccccc') == ['c', 'a', 'b']
  assert top_five_characters('hello world') == ['l', 'o', 'h', 'e', ' ']
  assert top_five_characters('aabbbccccddddeeeee') == ['e', 'c', 'd', 'b', 'a']
output: def top_five_characters(s):
      from collections import Counter
      counts = Counter(s)
      most_common = counts.most_common(5)
      return [char for char, count in most_common]