[lxml] Broken EXSLT link in docs

Hello, I think the EXSLT link here: https://lxml.de/xpathxslt.html#regular-expressions-in-xpath or source here: https://github.com/lxml/lxml/blob/9818374770aedc96f8f1e77943f45dea8e7fb4a8/d... should change from http://www.exslt.org/ to https://exslt.github.io/ or some other valid URL. Happy to provide a PR but the change seems trivial enough… Cheers, Jens -- Jens Tröger https://savage.light-speed.de/