input: Write a function to find the maximum value in a list.
  assert find_max([2, 4, 8, 9, 10]) == 10
  assert find_max([1, 3, 5, 7, 9]) == 9
  assert find_max([10, 20, 5, 15, 25]) == 25
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