input: Write a python function to find the key with the maximum value in a dictionary.
  assert max_key_dict({hello:1, goodbye:2, test:3}) == 'test'
  assert max_key_dict({a:10, b:20, c:5}) == 'b'
  assert max_key_dict({1:100, 2:200, 3:300}) == 3
output: def max_key_dict(d):
      return max(d, key=d.get)