Lock the table before checking uniqueness of coupon number

Closes #13011
(grafted from eb6278d920c1c9dcbb62dd8e023f63c7b0388173)

--HG--
branch : 7.0

modules/sale_promotion_coupon/sale.py

@@ -155,6 +155,7 @@ class PromotionCouponNumber(DeactivableMixin, ModelSQL, ModelView):
     def check_unique(cls, numbers, field_names=None):
         if field_names and not (field_names & {'number', 'coupon'}):
             return
+        cls.lock()
         duplicates = []
         for sub_numbers in grouped_slice(numbers):
             domain = ['OR']