changing some classic division to exact division in ascii_art of trees

[Frédéric Chapoton]

some problems (about future transition to py3) can be found using

export PYTHONIOENCODING="utf-8"
python -Qwarnall $(which sage-runtests) src/sage/combinat/*.py 

let us try to correct a few of them.

see also #18659 and #15995

[Frédéric Chapoton]

New commits:

​1ab50b7

changing some division into exact division in ascii_art

[Travis Scrimshaw]

Since you're changing it, you might as well do:

-while len(roots):
+while roots:

Once you make those changes, you can set it to a positive review.

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​7f7a7b5

trac 20468 reviewer's suggestion

[Frédéric Chapoton]

thanks Travis

[Frédéric Chapoton]

sequel in #20471

[Jeroen Demeyer]

General suggestion: add from __future__ import division on top of modules which have been fully converted. That way we know for sure that there are no old-style divisions left in that module.