input: Write a python function to sort a dictionary by its keys.
  assert sort_dict_by_keys({'b': 2, 'a': 1, 'c': 3}) == {'a': 1, 'b': 2, 'c': 3}
  assert sort_dict_by_keys({3: 'three', 1: 'one', 2: 'two'}) == {1: 'one', 2: 'two', 3: 'three'}
  assert sort_dict_by_keys({'banana': 3, 'apple': 4, 'pear': 1, 'orange': 2}) == {'apple': 4, 'banana': 3, 'orange': 2, 'pear': 1}
output: def sort_dict_by_keys(d):
      return dict(sorted(d.items()))