[FIX] base, web_editor, website: add missing encoding on etree.tostring

Since lxml 4.5.0 that use now use libxml2 2.9.10, without it, it can crash.

Here sample of content that is broken:
https://drive.google.com/file/d/1gB2-jl4fabHscLjH9OZc4WZgtSEbCVj7/view?usp=sharing



note: this commit is to merge 3 changes of #64526

opw-2428617
opw-2428664

closes odoo/odoo#66866

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>