Remove redundant code in backwardcompat

[Ivoz]

I'd imagine six duplicates a lot of the code present here, especially for the py2/3 differences, so it would be good to delete as much of this as possible and replace its usages with those from six. The less code the around the better, IMHO. Just marking as a bit of low hanging fruit to get to.

[qwcode]

btw, a persnickety thing, but I wouldn't mind moving the backwardcompat.py package to simpler named compat.py module as well.

[dstufft]

Agreed on all counts.