ERROR: Could not install packages due to an OSError: HTTPSConnectionPool

错误异常分析：ERROR: Could not install packages due to an OSError: HTTPSConnectionPool(host='files.pythonhosted.or 原因是安装flask框架模块，必须增加flask模块到requirements.txt中，因此执行如下命令:

python -m pip install -r requirements.txt

然后执行：pip install flask  安装成功