Done through various 2to3 fixers (zope.fixers, modernize, future) and also
contain manual changes. This is a single commit so that we have a clearer
picture of how code converted with my2to3 should look like.

Except straightforward @implementer decorator 2to3 fixer, only product/ folder
was considered as the goal was to be able to create an ERP5Site.

* Use @implementer decorator introduced in zope.interface 3.6.0 (2010):

  The implements syntax used under Python 2.X does not work under 3.X, since it
  depends on how metaclasses are implemented and this has changed. Instead it
  now supports a decorator syntax (also under Python 2.X).

  Applied thanks to 2to3 `zope.fixers` package.

* Use `six.moves` rather than `future` install_aliases() feature because the
  latter use unicode_literals and "wraps" module aliases so that unicode() are
  returned for text rather than str() (Python2 standard library). This notably
  breaks BusinessTemplate code which uses urllib quote() for filesystem paths...

* No more unbound methods in python3 so use six.get_unbound_function().

* dict.(iteritems,iterkeys,itervalues)() => six.\1(dict) thanks to `dict_six`
  2to3 fixer from `modernize`:
  $ python-modernize -w -f dict_six product/

* Manually make sure that dict.{items,values,keys}() returns a real list when
  needed rather than a dict_{items,values,keys} when it is latter modified. By
  default, 2to3 blindly does list(dict.{items,values,keys}()) which is not
  acceptable from performances point of view. With my2to3, this will be possible
  to handle such case automatically.