centos6.8编译安装python2.7之后，使用pip报错：pip cannot confirm SSL certificate: SSL module is not available
解决方法：
安装openssl-devel 后重新编译安装python
ref：
https://stackoverflow.com/questions/25981703/pip-install-fails-with-connection-error-ssl-certificate-verify-failed-certi
https://stackoverflow.com/questions/44290926/pip-cannot-confirm-ssl-certificate-ssl-module-is-not-available
https://stackoverflow.com/questions/41328451/ssl-module-in-python-is-not-available-when-installing-package-with-pip3