Remove arguments in super() calls.

They are no more needed in Python3 and usually result in unexpected
behavior when wrongly used (like e.g. the one introduced in
764cacc091 and solved meanwhile by
refactorization).

mixin.py

@@ -47,7 +47,7 @@ class EdocumentMixin(object):
         Returns:
             record : model party.address
         """
-        result = super(EdocumentMixin, self).seller_trade_address
+        result = super().seller_trade_address
         if not result:
             raise UserError(gettext(
                 'edocument_xrechnung.msg_no_seller_address',
@@ -75,7 +75,7 @@ class EdocumentMixin(object):
                 if self.invoice and self.invoice.party
                 else '-'))
 
-        result = super(EdocumentMixin, self).buyer_trade_address
+        result = super().buyer_trade_address
         if result and not result.country:
             raise UserError(gettext(
                 'edocument_xrechnung.msg_no_address_country',