Use aptitude instead of apt-get. It is more intelligent. It not only will handle downgrading conflicting packages for you, but will make a series of recommendations asking you which of many possible suggested working scenarios you would like.
sudo aptitude install myNewPackage
If you don't have aptitude on your machine yet, get it with
sudo apt-get install aptitude
自动提出解决方案
参考：https://askubuntu.com/questions/363200/e-unable-to-correct-problems-you-have-held-broken-packages
安装pip时候 no model named setuptools
sudo apttitude install -y python-setuptools