[entity] improve deprecation messages

[devtools] use self._parse so AssertionError is properly raised instead of lxml error (test failure introduced in 6711f78c18be)

[testlib] fix page validator selection. Closes #2869456 * use a validator that checks XML well-formness for the html5 doctype (as a convenient side-effect, it does not choke on e.g. a canvas tag like the HTMLParser does) * use a DTD validator if there is an xml declaration * else use the HTMLValidator

[testlib] introduce a validator that check xml-well formness This validator simply check XML is well formed and accept any entities (think HTML defines much more entities than bare XML)

[view] return HTML5 doctype. Closes #2869426 We dropped xhtml support in 3.17 but we kept providing documents using the XHTML strict doctype, while our content wasn't conform to the DTD since we dropped proper namespaces definition as well as declaration of cubicweb DTD extensions.

[testlib] repreprocess content so contextual display has a chance to show the proper line. Closes #2869481 the context is calculated using position information relative to a preprocessed content, not the original source

[web request] drop no more necessary request.document_surrounding_div method This is awful and no more necessary since we dropped xhtml support

[testlib] unspaghettify Validator / PageInfo api

[testlib] update htmlparsers.VALMAP: stop using SaxOnlyValidator and add an entry for html

[testlib] SaxOnlyValidator is deprecated, use XMLValidator instead