Add methods for python3 iterator

xmlCoreDepthFirstItertor and xmlCoreBreadthFirstItertr only
implement a python2-compatible iterator interface. The next()
method has been changed to __next__(). An alias has been
defined to keep python2 compatibility.
Merge branch 'master' of git://git.gnome.org/libxml2 into os400
os400: make-src.sh: create physical file with target CCSID