当出现类似这类错误：
The following packages have unmet dependencies:
python-dev : Depends: python (= 2.7.5-5ubuntu3) but 2.7.12-1~16.04 is to be installed
Depends: libpython-dev (= 2.7.5-5ubuntu3) but it is not going to be installed
Depends: python2.7-dev (>= 2.7.5-1~) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
这是由于安装包的版本太新，其依赖包和旧版本冲突导致。
网上常见解决方法：apt-get -f install，but it doesn't work for me，或者说使用aptitude的，试了下很慢。
后来我把阿里源换回Ubuntu原生的源就可以安装了，因为阿里源的包太新。