[FIX] account: do not use optional library

stdnum is not a mandatory dependency
It is often present as required to install the base_vat module but
having a system with account without the dependency should be
possible.

As the calc_check_digits is pretty small, it is easy to extract

Forward port of 6091be6556d to 13.0

closes odoo/odoo#40265

X-original-commit: ef245e14
Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>