ERROR: Could not find a version that satisfies the requirement flake8

安装python的过程中出现错误：
ERROR: Could not find a version that satisfies the requirement flake8 (from versions: none)
ERROR: No matching distribution found for flake8

ERROR: Could not find a version that satisfies the requirement yapf (from versions: none)
ERROR: No matching distribution found for yapf

解决方案：

pip install flake8 -i http://pypi.doubanio.com/simple/ --trusted-host pypi.doubanio.com
pip install yapf -i http://pypi.doubanio.com/simple/ --trusted-host pypi.doubanio.com

原因：
干脆不使用https://pypi.python.org网站了，我们用一个镜像网站，那边是：

http://pypi.doubanio.com/simple/