input: Write a python function to find the maximum number in a list.
  assert max_number([12, 5, 23, 1, 11]) == 23
  assert max_number([3, 6, 8, 2, 4, 10]) == 10
  assert max_number([-1, -5, -3, -4, -2]) == -1
output: def max_number(lst):
      if not lst:
          return None
      max_num = lst[0]
      for num in lst:
          if num > max_num:
              max_num = num
      return max_num