input: Write a python function to find the maximum number in a list.
  assert find_max([1, 4, 7, 3]) == 7
  assert find_max([-1, -4, -7, -3]) == -1
  assert find_max([10, 20, 30, 5, 25]) == 30
output: def find_max(lst):
      if not lst:
          return None
      max_num = lst[0]
      for num in lst:
          if num > max_num:
              max_num = num
      return max_num