input: Write a python function to find the character with the maximum occurrence in a string.
  assert max_occurrence("hello") == 'l'
  assert max_occurrence("teststring") == 't'
  assert max_occurrence("aabbccddeeffg") == 'a'
output: def max_occurrence(s):
      from collections import Counter
      count = Counter(s)
      return max(count, key=count.get)