python保存文件出错怎么解决？

# -*- coding: utf-8 -*-

import xlwt, datetime

# 创建一个workbook 设置编码

workbook = xlwt.Workbook(encoding = 'utf-8')

# 创建一个worksheet

worksheet = workbook.add_sheet('My Worksheet')

# 写入excel

worksheet.write(0, 0, label = 'this is test')

# 保存

suffix= datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")

workbook.save('./test_%s.xls' % suffix)

Traceback (most recent call last):

  File "C:/Users/HP/PycharmProjects/Github_From_PyCharm/test_xlwt.py", line 14, in <module>

    workbook.save('./test_%s.xls' % suffix)

  File "C:\Users\HP\PycharmProjects\test\venv\lib\site-packages\xlwt\Workbook.py", line 710, in save

    doc.save(filename_or_stream, self.get_biff_data())

  File "C:\Users\HP\PycharmProjects\test\venv\lib\site-packages\xlwt\CompoundDoc.py", line 262, in save

    f = open(file_name_or_filelike_obj, 'w+b')

OSError: [Errno 22] Invalid argument: './test_2020-02-19 21:12:13.xls'