input: Write a python function to split a dictionary's keys into two equally-sized sets.
  assert split_dict({1: 'a', 2: 'b', 3: 'c', 4: 'd', 5: 'e', 6: 'f'}) == ([1, 2, 3], [4, 5, 6])
  assert split_dict({10: 'x', 20: 'y', 30: 'z', 40: 'w'}) == ([10, 20], [30, 40])
  assert split_dict({1: 'a', 2: 'b'}) == ([1], [2])
output: def split_dict(d):
      keys = list(d.keys())
      mid = len(keys) // 2
      return (keys[:mid], keys[mid:])