[test] Use cubicweb-file < 2.0.0 From version 2.0.0, cubicweb-file moved to new-style layout (Python package) but we still expect it to be an old-style cube in CubicWeb's tests (WebconfigTC in web/test/unittest_webconfig.py).

[py3] Decode subprocess stdout in toolsutils.show_diffs() On Python 3, subprocess' pipes are bytes stream so we must decode them before printing. Without this change, diff of configuration files (e.g. all-in-one.conf as displayed upon instance upgrade) will rendered as a single line on Python 3.