以下记录我在pip install某package时遇到的几种报错，幸运地，这些雷在安装PyQt5时全踩中了，那就以它为例。
1 三种报错
Case 1：无法获取URL，证书有问题
WARNING: Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError(SSLError(1, '[SSL: KRB5_S_TKT_NYV] unexpected eof while reading (_ssl.c:1091)'))': /simple/pyqt5/
Could not fetch URL https://pypi.org/simple/pyqt5/: There was a problem confirming the ssl certificate: HTTPSConnectionPool(host='pypi.org', port=443): Max retries exceeded with url: /simple/pyqt5/ (Caused by SSLError(SSLError(1, '[SSL: KRB5_S_TKT_NYV] unexpected eof while reading (_ssl.c:1091)'))) - skipping
ERROR: Could not find a version that satisfies the requirement PyQt5 (from versions: none)
ERROR: No matching distribution found for PyQt5
Case 2：没有匹配的发行版
Looking in indexes: http://pypi.douban.com/simple/
WARNING: The repository located at pypi.douban.com is not a trusted or secure host and is being ignored. If this repository is available via HTTPS we recommend you use HTTPS instead, otherwise you may silence this warning and allow it anyway with '--trusted-host pypi.douban.com'.
ERROR: Could not find a version that satisfies the requirement PyQt5 (from versions: none)
ERROR: No matching distribution found for PyQt5
Case 3：找不到合适的版本
Looking in indexes: http://pypi.douban.com
ERROR: Could not find a version that satisfies the requirement PyQt5 (from versions: none)
ERROR: No matching distribution found for PyQt5
2 解决办法
查阅了很多资料，借鉴了很多博客，综合起来就是在命令后面加上：
-i http://pypi.douban.com/simple --trusted-host=pypi.douban.com
加在一起也即：
pip3 install PyQt5 -i http://pypi.douban.com/simple --trusted-host=pypi.douban.com
更换源的同时说明信任该源。
3 说明
经测试，以上方法适用于自己电脑或者亚马逊云服务器等，其他平台不太清楚。
但如果是在Kaggle上，可以参考这个链接。
其他情况欢迎在评论区留言。