input: Write a function to find the maximum number in a list.
  assert find_max([1, 9, 5, 18, 21, 2]) == 21
  assert find_max([3, 6, 2, 8, 5, 10]) == 10
  assert find_max([-1, -5, -3, -10]) == -1
output: def find_max(lst):
      return max(lst)