An unicode encoding error was being raised in
test_AccountingTransaction_getListBoxColumnList_item_column in Python 2,
when the `html` variable was being passed to the `StringIO`
constructor. The error was only reproducible when running the tests in
the CLI.

The problem was that we were using cStringIO module in Python 2, and the
string فارسی (farsi) appears in the HTML (probably inside a language
picker). cStringIO cannot parse unicode characters that are non-ASCII.
The solution is to use "six.StringIO", which uses the (slower,
but unicode-compatible) StringIO module in Python 2.

fixup! fixup! fixup! fixup! fixup! XMLExportImport: more support pickle protocol 3 🚧 ( repair python2 )

This is no longer needed now, maybe since Zope4. Which is a good thing
because this option cannot be used on python3

See merge request nexedi/erp5!1912

fixup! fixup! fixup! fixup! fixup! XMLExportImport: more support pickle protocol 3 🚧 ( repair python2 )

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.

!1751 (comment 203735)