使用pip安装pymysql出错;Could not find a version that satisfies the requirement cryptography (from pymysql) (from versions: ) No matching distribution found for cryptography (from pymysql)

今天使用pip安装pymysql时出现如下错误：

Could not find a version that satisfies the requirement cryptography (from pymysql) (from versions: )
No matching distribution found for cryptography (from pymysql)

使用pip安装其他模块时也可能出现上述错误，主要原因是网络的问题，需要使用国内的镜像源来加速，比如豆瓣源

因此命令可以使用：

pip install pymysql -i http://pypi.douban.com/simple/ --trusted-host pypi.douban.com

或者

pip3 install pymysql -i http://pypi.douban.com/simple/ --trusted-host pypi.douban.com

即可成功安装。