pip升级的错误

AttributeError: ‘NoneType’ object has no attribute ‘bytes’

在PyCharm创建的Virtualenv环境下，使用pip安装包时，老是提示需要升级pip，具体提示信息为：

You are using pip version 10.0.1, however version 19.1 is available.
You should consider upgrading via the 'python -m pip install --upgrade pip' command.

而当执行完 python -m pip install --upgrade pip 后确报如下错误：

AttributeError: 'NoneType' object has no attribute 'bytes'

解决方法如下:

#这个方法我试了是可以的
1、python -m pip install -U --force-reinstall pip 

2、python -m pip install -U pip

3、easy_install -U pip