No module named pip._internal.cli.main Mac 下更新pip到20.1.1

1.出現以下提醒

WARNING: You are using pip version 19.2.3, however version 20.1.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.

2.更新出錯

3.重新升級安裝pip

python3 -m pip install --upgrade pip

4.安裝成功