input: Write a function to find the maximum value in a list.
  assert find_max_value([1, 5, 9, 4, 8]) == 9
  assert find_max_value([10, 20, 30, 40, 50]) == 50
  assert find_max_value([-1, -5, -3, -10]) == -1
output: def find_max_value(lst):
      if not lst:
          return None
      max_value = lst[0]
      for value in lst:
          if value > max_value:
              max_value = value
      return max_value