在安装sklearn时报错如下：
ERROR: Could not find a version that satisfies the requirement threadpoolctl>=2.0.0 (from scikit-learn->sklearn) (from versions: 1.0.0, 1.1.0)
ERROR: No matching distribution found for threadpoolctl>=2.0.0 (from scikit-learn->sklearn)
个人以为是threadpoolctl包的问题。
实际上经过查找是网络不好，也就是源出了问题，我的源是 https://pypi.mirrors.ustc.edu.cn/simple/
解决办法：pip install sklearn -i http://pypi.douban.com/simple/ --trusted-host pypi.douban.com