input: Write a python function to find the key with the highest value in a dictionary.
  assert highest_value({'a': 10, 'b': 20, 'c': 30}) == 'c'
  assert highest_value({'x': 5, 'y': 15, 'z': 10}) == 'y'
  assert highest_value({'key1': 1, 'key2': 2, 'key3': 3}) == 'key3'
  assert highest_value({'apple': 100, 'banana': 50, 'cherry': 150}) == 'cherry'
output: def highest_value(d):
      return max(d, key=d.get)