input: Write a function to find the maximum number in a list.
  assert find_max([9, 4, 2, 7, 5]) == 9
  assert find_max([1, 2, 3, 4, 5]) == 5
  assert find_max([-10, -20, -5, -1]) == -1
output: def find_max(lst):
      return max(lst)