运行pip install Flask时，出现此错误
Collecting flask
Could not fetch URL https://pypi.python.org/simple/flask/: There was a problem confirming the ssl certificate: [SSL: TLSV1_ALERT_PROTOCOL_VERSION] tlsv1 alert protocol version (_ssl.c:661) - skipping
Could not find a version that satisfies the requirement flask (from versions: )
No matching distribution found for flask