[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.
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.