input: Write a python function to find the most frequent character in a given string.
  assert most_frequent_char("Python Program") == 'P'
  assert most_frequent_char("aabbccddeeff") == 'a'
  assert most_frequent_char("hello world") == 'l'
output: def most_frequent_char(s):
      from collections import Counter
      count = Counter(s)
      return max(count, key=count.get)