input: Write a python function to find the maximum value in a list.
  assert find_max([1, 4, 6, 8, 3, 9, 5]) == 9
  assert find_max([10, 15, 20, 25, 30]) == 30
  assert find_max([-10, -20, -5, -1]) == -1
output: def find_max(lst):
      if not lst:
          return None
      max_value = lst[0]
      for num in lst:
          if num > max_value:
              max_value = num
      return max_value