Comment 7 for bug 1882606

Here's a fix:
https://github.com/lxml/lxml/commit/dd2d80a416e0aa5e177a723bcd571acf83a4c06a

I think it's not a backward compatibility issue. If users requested comments/PIs *not* to be removed, then they'll now have them preserved. That's it. Given that removal was the default, users explicitly had to express their intent to keep them.