Could not fetch URL https://pypi.python.org/simple/six/: There was a problem confirming the ssl certificate: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:600) - skipping
Could not find a version that satisfies the requirement six
经过网上查阅资料，发现是python版本的问题，高版本的Python会默认SSL。
解决方法：
pip install --trusted-host pypi.python.org 安装包的名字
