pip更新报错：Requirement already up-to-date

pip是一个 Python 包安装与管理工具，非常好用。

执行 pip install --upgrade pip 报错：
DEPRECATION: Python 3.4 support has been deprecated. pip 19.1 will be the last one supporting it. Please upgrade your Python as Python 3.4 won't be maintained after March 2019 (cf PEP 429).
Requirement already up-to-date: pip in /Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages (19.1.1)

解决办法：强制更新pip
执行：python -m pip install --upgrade pip