pip升级到7.0以后，在使用http镜像进行包安装及升级的时候往往会有如下提示：

Collecting requests
  The repository located at pypi.douban.com is not a trusted or secure host and is being ignored. If this repository is available via HTTPS it is recommended to use HTTPS instead, otherwise you may silence this warning and allow it anyways with '--trusted-host pypi.douban.com'.
  Could not find a version that satisfies the requirement requests (from versions: )
No matching distribution found for requests

解决办法如下：
1.安装时加入 --trusted-host pypi.python.org参数：
 pip --trusted-host pypi.python.org install requests
2.在pip.conf中加入trusted-host选项，该方法是一劳永逸

[global]
timeout = 6000
index-url = http://pypi.douban.com/simple/
[install]
use-mirrors = true
mirrors = http://pypi.douban.com/simple/
trusted-host = pypi.douban.com