pip install报错Could not fetch URL https://pypi.org/simple/pip/: There was a problem confirm

pycharm使用pip install xxx安装包时，一直报错：

WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=No
ne)) after connection broken by 'SSLError(SSLEOFError(8, 'EOF occurred in violation
 of protocol (_ssl.c:1123)'))': /simple/flask/
WARNING: Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=No
ne)) after connection broken by 'SSLError(SSLEOFError(8, 'EOF occurred in violation
 of protocol (_ssl.c:1123)'))': /simple/flask/
ERROR: Could not find a version that satisfies the requirement xxx (from versions
: none)
ERROR: No matching distribution found for xxx
Could not fetch URL https://pypi.org/simple/pip/: There was a problem confirming th
e ssl certificate: HTTPSConnectionPool(host='pypi.org', port=443): Max retries exce
eded with url: /simple/pip/ (Caused by SSLError(SSLEOFError(8, 'EOF occurred in vio
lation of protocol (_ssl.c:1123)'))) - skipping

原因：pypi.python.org在国内被墙了，无法从python官网url下载。

解决办法：
更改为国内的镜像源：（xxx为安装的包）

pip install xxx -i http://pypi.douban.com/simple --trusted-host pypi.douban.com
pip3 install -i https://pypi.tuna.tsinghua.edu.cn/simple xxx

豆瓣(douban) http://pypi.douban.com/simple/

阿里云 http://mirrors.aliyun.com/pypi/simple/

清华大学 https://pypi.tuna.tsinghua.edu.cn/simple/

中国科技大学 https://pypi.mirrors.ustc.edu.cn/simple/

中国科学技术大学 http://pypi.mirrors.ustc.edu.cn/simple/

注：还尝试了一种方法，是加上–trusted-host

执行 pip install xxx -i https://pypi.org/simple --trusted-host pypi.org 但是仍然报错

这次应该url对了，但是还是一直WARNING