[skeleton] use python3 flake8
In tox "flake8" environment, use python 3 virtualenv, so we can check for
python3 incompatible syntax.
3.26 (1 February 2018)
======================
New features
------------
* For ``pyramid`` instance configuration kind, logging is not handled anymore
by CubicWeb but should be configured through ``development.ini`` file
following https://docs.pylonsproject.org/projects/pyramid/en/latest/narr/logging.html.
Backwards incompatible changes
------------------------------
* CubicWebConfiguration method 'shared_dir' got dropped.