使用pip install直接安装时下载速度慢，且容易超时（Read timed out）
pip install opencv-python
错误如下：
ERROR: Exception:
socket.timeout: The read operation timed out
(原始是因为直接下载会被墙)
解决方案是换用国内镜像清华源：
pip install https://pypi.tuna.tsinghua.edu.cn/simple
此时如果不出意外则会提示安装成功
Successfully installed numpy xxxxx
如果还继续出现如下之类错误：
ERROR: Cannot unpack file C:\User…; charset=u
tf-8); cannot detect archive format
ERROR: Cannot determine archive format of C:\Users…
则再次键入：
pip install -i https://pypi.tuna.tsinghua.edu.cn/simple --trusted-host pypi.tuna.tsinghua.edu.cn opencv-python
则安装成功：
Successfully installed numpy-xxxxx opencv-python-xxxx
