Do not copy sale lines when copying purchase requests

Closes #12918
(grafted from 3a4693cd3e305eb2b2ae8d34c92ecef275ee9adc)

--HG--
branch : 7.0

modules/sale_supply/purchase.py

@@ -18,6 +18,12 @@ class Request(metaclass=PoolMeta):
     def _get_origin(cls):
         return super()._get_origin() | {'sale.sale'}
 
+    @classmethod
+    def copy(cls, requests, default=None):
+        default = default.copy() if default is not None else {}
+        default.setdefault('sale_lines')
+        return super().copy(requests, default=default)
+
     @classmethod
     def delete(cls, requests):
         pool = Pool()