Fix Python code: remove imports from `python-future`.

Fix Python code: remove imports from `python-future`.

I missed in 3d6ba937a5a5046b3d531c17691d5d1155daae71 that `future`
also provides the modules `past` and `builtins`.
Imports of `builtins` can simply be removed for Python-3-only code.
`past.old_div` had to be replaces by the "old" div-operatot `//`,
while `past.xrange` was unused.