input: Write a function to find the most common character in a string.
  assert most_common_char("hello") == 'l'
  assert most_common_char("test") == 't'
  assert most_common_char("character") == 'c'
  assert most_common_char("aabbcc") == 'a'
output: def most_common_char(s):
      from collections import Counter
      count = Counter(s)
      return max(count, key=count.get)