This module required `vatnumber` just to make sure upstream odoo behaved as expected.
However, upstream changed to using `stdnum` in c377e46da1, so that's what we should require here now.
Besides, tests were bringing false positives and negatives. Now they test the correct behavior.
@moduon MT-5599
* base_vat_optional_vies: Fix pass lower country code in _split_vat function to find parent function check_vat_'xx' instead of check_vat_'XX'
* base_vat_optional_vies: Convert to upper when write NIF into database
