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

see discussion in https://github.com/dmwm/CRABClient/pull/5137#discussion_r766750997 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 lot 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.