解决Windows下pip安装模块报错问题，以及python2和python3共存问题

解决Could not fetch URL https://pypi.org/simple/pip/: There was a problem confirmingthe ssl certificate: HTTPSConnectionPool(host=‘pypi.org’, port=443): Max retries exceeded with url: /simple/pip/ (Caused by SSLError(SSLError(1, u’[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:661)’),)) - skipping

解决python2和python3共存问题，转自：
https://www.jianshu.com/p/e30c52f27dad
解决上面pip安装模块报错信息，转自：
https://blog.csdn.net/qq_34504481/article/details/81910253