input: Write a function to find the maximum value in a list of integers.
  assert max_value([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])==10
  assert max_value([10, 20, 45, 67, 84, 93])==93
  assert max_value([5, 7, 9, 8, 6, 4, 3])==9
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