input: Write a function to find the maximum value in a list.
  assert find_max_value([1, 3, 2, 8, 5, 10, 7]) == 10
  assert find_max_value([-1, -5, -3, -4, -2, -10]) == -1
  assert find_max_value([100, 200, 150, 300, 250]) == 300
output: def find_max_value(lst):
      if not lst:
          return None
      max_value = lst[0]
      for num in lst:
          if num > max_value:
              max_value = num
      return max_value