input: Write a python function to find the most frequent character in a string.
  assert most_frequent("Programmers") == 'r'
  assert most_frequent("Mississippi") == 'i'
  assert most_frequent("Hello") == 'l'
output: def most_frequent(s):
      from collections import Counter
      count = Counter(s)
      most_common_char, _ = count.most_common(1)[0]
      return most_common_char