[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#66883

X-original-commit: 38983d123d171ac3cc1efacd99d9776f0ed08734
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>

addons/web_editor/models/ir_ui_view.py

@@ -335,7 +335,7 @@ class IrUiView(models.Model):
         name = self._find_available_name(name, used_names)
 
         # html to xml to add '/' at the end of self closing tags like br, ...
-        xml_arch = etree.tostring(html.fromstring(arch))
+        xml_arch = etree.tostring(html.fromstring(arch), encoding='utf-8')
         new_snippet_view_values = {
             'name': name,
             'key': full_snippet_key,

addons/website/models/website_snippet_filter.py

@@ -71,7 +71,7 @@ class WebsiteSnippetFilter(models.Model):
             is_sample=is_sample,
             to_generic=self._to_generic,
         )).decode('utf-8')
-        return [ET.tostring(el) for el in ET.fromstring('<root>%s</root>' % content).getchildren()]
+        return [ET.tostring(el, encoding='utf-8') for el in ET.fromstring('<root>%s</root>' % content).getchildren()]
 
     def _prepare_values(self, limit=None, search_domain=None):
         """Gets the data and returns it the right format for render."""

odoo/addons/base/models/qweb.py

@@ -372,7 +372,7 @@ class QWeb(object):
 
         if isinstance(document, etree._Element):
             element = document
-            document = etree.tostring(document)
+            document = etree.tostring(document, encoding='utf-8')
         elif not document.strip().startswith('<') and os.path.exists(document):
             element = etree.parse(document).getroot()
         else: