Unable to locate package python3-pip

#1 Unable to locate package python3-pip
##1.1 问题： 
执行： sudo apt-get install python3-pip时，报错：
Reading package lists... Done
Building dependency tree Reading state information... Done
E: Unable to locate package python3-pip

##1.2 解决办法：
vi /etc/apt/sources.list  

Then add universe category at the end of each line:

deb http://archive.ubuntu.com/ubuntu bionic main universe
deb http://archive.ubuntu.com/ubuntu bionic-security main universe
deb http://archive.ubuntu.com/ubuntu bionic-updates main universe

sudo apt-get update
sudo apt-get install -y python3-pip

##1.3 参考：
https://stackoverflow.com/questions/52394543/pip-install-problem-with-ubuntu-18-04-and-python-3-6-5