[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.