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.

* past.builtins:
  portal_skins/erp5_core/Base_sortObjectListLikeSQL:0:2:Redefining built-in 'cmp' (redefined-builtin)
* six and six.moves imports:
  + portal_skins/erp5_core/Base_updateListboxSelection:0:2:Redefining built-in 'basestring' (redefined-builtin)
  + portal_skins/erp5_core/ERP5Site_checkDataWithScript:20:2:Redefining built-in 'xrange' (redefined-builtin),

Use the latter form that works on both. On non-PythonScript code, this is of
course not an issue but Python2 PythonScript does not have a __code__ properties
and requires a patch on Shared.DC.Scripts.Signature (applied by SlapOS recipe)
so that FuncCode() object is set to not only func_code but also __code__.

And also disable pylint patch as pylint version with Python3 support is not
supported yet by ERP5...

* Add functions to properly convert/handle bytes/str/unicode (from
  slapos.util) (stricter than six.ensure_{text,str}()).
* Python3 {keys,values,items}() now returns dict_{keys,values,items} rather than
  real list() so add ensure_list() to create a list() when it is latter modified
  but do nothing on Python2 for performance sake.

  File "product/TimerService/timerserver/TimerServer.py", line 71, in run
    s.send('GET / HTTP/1.1\r\n\r\n')
TypeError: a bytes-like object is required, not 'str'

And Zope4 now mandates it to be a string ({Shared.DC.ZRDB.TM.TM,ITransactionManager}.sortKey()).

This changes the API to implement similar feature in Python3, but this should
not be an issue because Interactor are only for non-ERP5 object...

py3: AccessControl users_with_local_roles() returns a dict_keys() instead of a subscriptable object.

Submitted upstream:
  https://github.com/zopefoundation/AccessControl/pull/128

py3: DCWorkflow is python2-only so make ERP5Site creation work on Python3 WITH_LEGACY_WORKFLOW=False.

ERP5 Workflow implementation should ideally not depend on DCWorkflow at all so
that legacy DCWorkflow can be disabled for Python2 but this would require much
more work.

Mainly for maintenance sake and to update the code before porting it to python3.

See merge request nexedi/erp5!1611

using json in 37b795fb was wrong,
because not all arguments can be converted to json (for example
DateTime instances)

This also enable the format using pprint.pformat for args, instead
of a simple str that was done in ActivityTool_getMessageList.

See merge request nexedi/erp5!1600

   The portal_skins was choosen to store the images to use proper http
cache.

   It is really not required, so we follow up the same as google oauth.

See merge request nexedi/erp5!1607

See merge request nexedi/erp5!1608

When spawning an activity, store the current security context's user in
the Message object itself, so the activity security context can be
re-created with the same security during activity execution.
This allows a user to be modified (different groups, global roles, maybe
removed altogether) after they spawned activities and before these activities
could run.
It also means that any temporary custom group or global role granted to
that user (by a privilege elevation mechanism out of the scope of this
change) will still be effective during the activity execution.
This follows the principle that
  foo.activate(...).bar(...)
should be equivalent to its "immediate execution" version
  foo.bar(...)
by ensuring that the security context of the activity is the same as the
one which was applied to the code which spawned that activity,
independently of any intermediate configuration change - hence improving
(deferred and fragmentary) transaction isolation.

This also removes the need to look the user up, then looking up their
assignments (and other documents involved in group computation), etc,
saving the cost of these calls.

Also, remove redundant user_name argument of Message.changeUser method.

The action was removed in 6dce55b0 (dms: drop PDF thumbnail view, 2021-06-01)
but some CSS declarations were not removed.

Also remove some old commented out javascript that was referencing the
old PDF preview

When a validated component is edited, `mixin.component.RecordablePropertyMetaClass`
records the `text_content` property. This recorded property will be cleared afterwards
if no errors are found, but this causes differences in business template XML for each
component the first time they are edited.

This change is about removing the `_recorded_property_dict` when there are no property
recorded, which leads to slightly smaller pickles (and business template XML, because
they are also pickles).

See merge request nexedi/erp5!1605

This makes smaller pickles