Instead of using encode/decode is it better to use str2bytes and
bytes2str, so that the types in Python 2 are unchanged. More important,
this prevents the double encoding of bytes, which causes problems
when they have non-ASCII data encoded.

Errors were being raised for returning `None` in Python 2
(due to a missing else branch) and for re-encoding bytes
in Python 2.

I changed the code to use str2bytes in the cases where bytes
are expected.

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)