so that Tester methods will have some __roles__ as Getter.

because str(float) result is different between py2/py3.

* py2
>>> str(-0.8999999999999999)
'-0.9'

* py3
>>> str(-0.8999999999999999)
'-0.8999999999999999'

with ROUND_DOWN, we have a different behaviour between py2/py3, that caused failures in
erp5_simplified_invoicing:testTradeModelLine.

Here is what happened in _round(1.9999999999999998) in
bt5/erp5_simulation/DocumentTemplateItem/portal_components/document.erp5.FloatEquivalenceTester.py

* py2
decimal.Decimal(str(1.9999999999999998)).quantize(decimal.Decimal('0.000001'), 'ROUND_DOWN')
=> Decimal('2.000000')
(because str(1.9999999999999998) is '2.0')

* py3
decimal.Decimal(str(1.9999999999999998)).quantize(decimal.Decimal('0.000001'), 'ROUND_DOWN')
=> Decimal('1.999999')
(because str(1.9999999999999998) is '1.9999999999999998')

But ROUND_DOWN result of 1.9999999999999998 with 0.000001 precision should be 1.999999 thus py2 behaviour is wrong.

nexedi/erp5!1751 (comment 203735)

On python2, attributes of new style classes (classes, not instances)
were not properly validated. On python3, attributes of classes were not.

In current versions, sorted is allowed in RestrictedPython.

Also, our implementation of sorted was not correct with generators,
it consumed the generator by iterating on it a first time to check the
values.