pip install 报错 Could not fetch URL

Could not fetch URL https://pypi.python.org/simple/xxx/: There was a problem confirming the ssl certificate: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:749) - skipping
Could not find a version that satisfies the requirement xxx (from versions: )
No matching distribution found for xxx

resolve:

　　　　pip --trusted-host pypi.python.org install xxx