[py3] Use "utf-8" as input encoding for docutils in rest extension Docutils says that "unicode" is an unknown encoding on Python3. Not sure where "unicode" as an encoding comes (it's there since showtime) but "utf-8" seems more appropriate. From now on, rendering of CubicWeb's rst directive (e.g. ..winclude) works on Python 3. Accordingly, we extend wdoc's test to demonstrate this. For this we need to install docutils in tox's web environment.

[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.