input: Write a function to find the biggest number in a list.
  assert biggest_number([1, 5, 4, 9, 2]) == 9
  assert biggest_number([10, 20, 5, 40, 30]) == 40
  assert biggest_number([-1, -5, -3, -4]) == -1
output: def biggest_number(lst):
      return max(lst)