[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.

cubicweb source: source-is-missing cubicweb/web/data/jquery.flot.js line length is * characters (>512)