input: Write a python function to convert a dictionary to a sorted list of tuples by keys.
  assert dict_to_sorted_list({'b': 2, 'a': 1, 'c': 3}) == [('a', 1), ('b', 2), ('c', 3)]
  assert dict_to_sorted_list({'blue': 3, 'red': 1, 'green': 2}) == [('blue', 3), ('green', 2), ('red', 1)]
  assert dict_to_sorted_list({'dog': 4, 'cat': 2, 'bird': 3}) == [('bird', 3), ('cat', 2), ('dog', 4)]
output: def dict_to_sorted_list(d):
      return sorted(d.items())