w3lib 在pypi网站上的版本为1.21.0
https://pypi.org/project/w3lib/

使用以下命令安装之后：

pip install w3lib

发现安装的版本为：

w3lib==1.19.0

通过以下命令可以查看到是可以升级的：

pip list --outdated

所以指定版本号来安装一下：

pip install w3lib==1.21.0
Collecting w3lib==1.21.0
Using cached https://files.pythonhosted.org/packages/6a/45/1ba17c50a0bb16bd950c9c2b92ec60d40c8ebda9f3371ae4230c437120b6/w3lib-1.21.0-py2.py3-none-any.whl
Requirement already satisfied: six>=1.4.1 in d:\python\python36\lib\site-packages (from w3lib==1.21.0) (1.11.0)
ERROR: scrapy 1.5.1 requires Twisted>=13.1.0, which is not installed.
Installing collected packages: w3lib
Found existing installation: w3lib 1.19.0
Uninstalling w3lib-1.19.0:
Successfully uninstalled w3lib-1.19.0
Successfully installed w3lib-1.21.0

w3lib的官方主页：
https://github.com/scrapy/w3lib

w3lib的文档地址：
https://w3lib.readthedocs.io/en/latest/