input: Write a python function to append a number to a list and return the updated list.
  assert append_number([2, 3, 4], 6) == [2, 3, 4, 6]
  assert append_number([], 1) == [1]
  assert append_number([1, 2], 3) == [1, 2, 3]
output: def append_number(lst, num):
      lst.append(num)
      return lst