环境：centos 6.5 + python2.7.13
问题：使用pip安装模块，总是包pip is configured with locations that require TLS/SSL, however the ssl module in Python is not available的错误。已知系统已经安装openssl、openssl-devel、pyopenssl等模块。请大神帮忙解答。
[root@www ~]# pip install six
pip is configured with locations that require TLS/SSL, however the ssl module in Python is not available.
Collecting six
Could not fetch URL https://pypi.python.org/simple/six/: There was a problem confirming the ssl certificate: Can't connect to HTTPS URL because the SSL module is not available. - skipping
Could not find a version that satisfies the requirement six (from versions: )
No matching distribution found for six
