错误

DEPRECATION: Python 2.7 reached the end of its life 
on January 1st, 2020. Please upgrade your Python as 
Python 2.7 is no longer maintained. pip 21.0 will 
drop support for Python 2.7 in January 2021. More 
details about Python 2 support in pip can be found 
at https://pip.pypa.io/en/latest/development
/release-process/#python-2-support pip 21.0 will 
remove support for this functionality.
 

原因分析：

默认的python环境为2.7，而pip在安装的时候会提示Python 2.7 reached the end of its life on January 1st, 2020.，因此我们需要更改运行环境。
换言之，此时我们用pip安装的就是在python3下安装的。

解决方案：

1.查看默认的python地址，返回 /usr/bin/python

$ which python

2.查看对应版本，返回Python 2.7.17

$ /usr/bin/python -V

3.类似操作，得到Python3的地址以及对应版本号

$ which python3
$ /usr/bin/python3

4.接下来我们写一个配置文件：

$ gedit ~/.bash_profile

往里面写入:

PATH="/usr/bin:${PATH}"
export PATH
alias python="/usr/bin/python3"

然后保存退出

$ source ~/.bash_profile

5.然后输入python运行的就是python3的环境了。