Remove more Python 2 specific constructs

[JulienCochuyt]

Follow-up of 68417d4
As Python 2 support as been dropped, we can get rid of more of its specific constructs.
Eg. we still carry on unnecessary long forms of super calls all over the place. These could easily get rid of by a simple sed script as we do not have IIRC cases where we indeed need them.
We could actually attempt to pass the whole code base through 2to3.

I guess we should wait until work on PR #21 is over as the merge might not be trivial there.

[JulienCochuyt]

super case actually done as of 4c1a80e and d2ead2a on for branch for PR #21.

Next: OrderedDict

[JulienCochuyt]

There are also six imports remaining in many places.