input: Write a python function to find the maximum number in a list.
  assert max_num([1, 2, 3, 4, 5]) == 5
  assert max_num([10, 20, 30, 40, 50]) == 50
  assert max_num([-1, -2, -3, -4, -5]) == -1
output: def max_num(lst):
      if not lst:
          return None
      max_value = lst[0]
      for num in lst:
          if num > max_value:
              max_value = num
      return max_value