input: Write a python function to find the maximum value in a list of integers.
  assert find_max_value([13, 33, 45, 3, 5, 17, 19, 21]) == 45
  assert find_max_value([1, 2, 3, 4, 5]) == 5
  assert find_max_value([-10, -20, -30, -5]) == -5
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