Ubuntu 20.04.3 “have unmet dependencies“

错误描述 — 在运行 sudo apt upgrade 时，出现以下报错：

The following packages have unmet dependencies:
 aptitude : Depends: libapt-pkg5.0 (>= 1.1) but it is not going to be installed
            Recommends: libparse-debianchangelog-perl but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

或者

The following packages have unmet dependencies:
 dpkg : Breaks: libapt-pkg5.0 (< 1.7~b) but 1.6.14 is to be installed
E: Broken packages

错误原因：

在更换国内源时，没有关注 Codename，Ubuntu 20.04.3 使用的是focal，却在使用国内阿里云源时，错误用了bionic版本的；

查询Codename，使用命令 lsb_release -a 查询；

lsb_release -a

处理方式：

通过修改镜像源, 使用正确的Codename.

执行以下命令：

sudo gedit /etc/apt/sources.list

 更换文件中的内容如下，并保存：

deb http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse