input: Write a python function to double the values of a dictionary.
  assert double_values({1:'a', 2:'b', 3:'c'}) == {1: 'aa', 2: 'bb', 3: 'cc'}
  assert double_values({4:'d', 5:'e', 6:'f'}) == {4: 'dd', 5: 'ee', 6: 'ff'}
  assert double_values({7:'g', 8:'h'}) == {7: 'gg', 8: 'hh'}
output: def double_values(d):
      return {k: v * 2 for k, v in d.items()}