参考：
Problem Confirming the SSL Certificate - OSX
OS X EI Captain 下解决 There was a problem confirming the ssl certificate 问题
在安装 matplotlib 时，出现以下错误：
python3 -mpip install matplotlib
Collecting matplotlib
Could not fetch URL https://pypi.python.org/simple/matplotlib/: There was a problem confirming the ssl certificate: [SSL: TLSV1_ALERT_PROTOCOL_VERSION] tlsv1 alert protocol version (_ssl.c:645) - skipping
Could not find a version that satisfies the requirement matplotlib (from versions: )
No matching distribution found for matplotlib
解决方法是使用以下方法更新pip：
curl https://bootstrap.pypa.io/get-pip.py | python3
2019.4