fix(account.invoice): replace private function `_parse_xml` with `parse_data`
Private functions are not intended to be called from outside of the class they belong to. We were
using _parse_xml function because module l10n_mx_ir_attachment_facturae_mx where not prividing a
better function to parse a XML string. On new version 4.0.1 there is a new public function called
parse_data that provides exactly the same features but is better for calling from outside of
ir.attachment.facturae class.