replace scattered ad-hoc py2/3 compatibility fixes with solid ones

[belforte]

see discussion in #5137 (comment) and below.
CRABClient was made python2/3 compatibly partly by dropping dependencies and simplifying, partly by inserting ad-hoc fixes here and there over last several months, e.g. with a log of if sys.version_info but not only.

For longer term stability and to minimize risks of regression, we should find all those hacks (!) and replace with more solid and uniform utilities as learnt during WMCore migration over last years.