[py3] Avoid setting attribute on Element instance in wdoc view This change is needed to make "wdoc" view work on Python 3. Indeed, before this patch, we used to set a "parent" attribute on Element instances, which is not permitted since Python 3.3 as xml.etree.ElementTree uses the C implementation by default. To get rid of this "parent" attribute, we rework the XML nodes processing logic to return and retrieve a node along with its parent in several place of the code. Namely, build_toc_index() now accepts the "parent" value for "node" and inserts it in the "index" dict (thus avoid `node.parent = <value>`). Respectively, every query to the "index" dict is updated to also retrieve the "parent" (thus avoiding `<value> = node.parent`). Later in InlineHelpView class, the "index" dict is bound to "tocindex" attribute so we adjust how it is queried in respective methods. Tests cubicweb/web/test/unittest_views_wdoc.py now pass on Python 3.

[test] Add a trivial test for "wdoc" view We just make sure it gets rendered without errors (which wasn't the case before the previous patch). It does not work on Python 3; this will be fixed in next changeset.

[py3] Open rst file in text mode in wdoc view Otherwise, UStringIO complains that it receives bytes.

[ext/rest] Drop "handle_io_errors" argument to docutils' FileInput This argument is deprecated and ignore in docutils since version 0.10. Besides, its default value is None which is the same as we passed.

[wdoc] Remove fallback about xml.etree module Module xml.etree exists since python 2.5.

[wdoc] Avoid usage of deprecated xlm elementree getchildren() method Use list(node) as suggested by the deprecation warning.

[style] Make cubicweb/web/views/wdoc.py flake8-clean

[py3] Use six.text_type instead of unicode in wdoc views

[views/startup] drop a duplicate rql query in manage view Counting entities in `nb` is already done above this code, regression introduced by a85db6fa

Merge with 3.26

Added tag 3.26.4, debian/3.26.4-1 for changeset e1c1af68db86

[pkg] Version 3.26.4