[FIX] account: dates can be datetime.date or strings

Open the customers followup report
on a line, click on its contextual menu (the down arrow next to invoice name)
click change expected payment date

Before this commit, there was a traceback, because the new date was a string
which has no strftime function

from 03c40e31 we learn that it can
either be a string or a date object, as Odoo in v12.0 supports this

After this commit, there is no traceback

OPW 1933953
OPW 1933979

closes odoo/odoo#30756