input: Write a function to return the first half of a list.
  assert first_half([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])==[0, 1, 2, 3, 4]
  assert first_half([10, 20, 30, 40, 50, 60, 70, 80, 90])==[10, 20, 30, 40]
  assert first_half([1, 3, 5, 7, 9, 11])==[1, 3, 5]
output: def first_half(lst):
      return lst[:len(lst) // 2]