also str representation of unicode character is different.

for same reason as the xhtml_style version, there is no problem with
having more than one listbox, see d2dedca3 (core: remove useless log
in Base_callDialogMethod, 2020-07-03)

This makes consistent method resolution order with python3 and reveal a
problem that relation field widgets render_pdf was not the expected one
on python3, solve this by defining explicitly the method.

roundDebitCredit does not raise in case of big difference since
696bc0d2 (2009-02-06 yo * Stop raising an exception, even if the
credit is not equal to the debit in a built delivery, in the post
processing script, because the builder may not blindly adopt prevision
when updating existing lines, thus it can be unbalanced., 2009-02-06)

This assertRaises syntax does nothing on python2 and was an error on
python2. Update the test to test the actual behavior