Done through various 2to3 fixers (zope.fixers, modernize, future) and 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 it
  is latter modified rather than a dict_{items,values,keys} (ensure_list()). 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.

* Replace cStringIO.StringIO() by six.moves.cStringIO() (a module alias for
  cStringIO.StringIO() on py2 and io.StringIO() on py3).

* Use six.text_type which maps to unicode() on py2 and str() on py3. This also
  makes a clearer difference between text and binary strings.

* Replace map()/filter() with lambda function by list comprehension (this has
  the benefit to avoid casting to list for py3 as it returns iterators).