[FIX] purchase: P3-style super call

3869cdf7d8 merged a fix which used a
P3-style super call, which is not compatible with Python 2.

While 11.0 is not really exactly officially supported on Python 2,
we originally decided not to break compatibility unless there were
very good reasons to do so.

addons/purchase/models/stock.py

@@ -190,6 +190,6 @@ class PushedFlow(models.Model):
     _inherit = "stock.location.path"
 
     def _prepare_move_copy_values(self, move_to_copy, new_date):
-        res = super()._prepare_move_copy_values(move_to_copy, new_date)
+        res = super(PushedFlow, self)._prepare_move_copy_values(move_to_copy, new_date)
         res['purchase_line_id'] = None
         return res