Clear requests when copying purchase lines

Closes #11989
(grafted from 92a2c368e3f7920227fb310ba8c8b5310338cdbc)

--HG--
branch : 6.0

modules/purchase_request/purchase.py

@@ -91,6 +91,12 @@ class Line(metaclass=PoolMeta):
             'invisible': ~Eval('requests'),
             })
 
+    @classmethod
+    def copy(cls, lines, default=None):
+        default = default.copy() if default is not None else {}
+        default.setdefault('requests')
+        return super().copy(lines, default=default)
+
     @classmethod
     def delete(cls, lines):
         pool = Pool()