input: Write a python function to find the maximum value in a list.
  assert max_value([23, 45, 67, 89, 13, 99, 19, 20]) == 99
  assert max_value([1, 2, 3, 4, 5]) == 5
  assert max_value([-1, -2, -3, -4]) == -1
output: def max_value(lst):
      if not lst:
          return None
      max_val = lst[0]
      for num in lst:
          if num > max_val:
              max_val = num
      return max_val