input: Write a function to find the maximum number in a list.
  assert find_max([8, 5, -1, 7, 2]) == 8
  assert find_max([1, 2, 3, 4, 5]) == 5
  assert find_max([-5, -2, -9, -1, -7]) == -1
output: def find_max(lst):
      return max(lst)