更新pip到最新版本失败，提示You are using pip version 19.0.3, however version 21.2.3 is available解决方法

更新pip到最新版本失败：

过程
1）打开CMD ，进入 C:\Program Files\Python37\Lib\site-packages
2）输入：C:\Program Files\Python37\Lib\site-packages>python -m pip install --user --upgrade pip
3）随后失败，提示warning：You are using pip version 19.0.3, however version 21.2.3 is available

解决方法
1）要先卸载pip， cmd输入命令行: python -m pip uninstall pip
2）然后重新执行上面步骤：C:\Program Files\Python37\Lib\site-packages>python -m pip install --user --upgrade pip
成功。