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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 wh...