[skeleton] use python3 flake8
In tox "flake8" environment, use python 3 virtualenv, so we can check for
python3 incompatible syntax.
missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/*/cubicweb/skeleton/debian/rules)embedded-javascript-library usr/share/cubicweb/cubes/shared/data/jquery.js