[views/uicfg] obvious bug detected by sonar

[style] Make toolsutils.py flake8-clean Run `autopep8 --max-line-length 100 -i cubicweb/toolsutils.py` and fixed unused imports.

[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.