Always surround WHERE clause with parentheses in EXCLUDE constraint

Closes #14449
(grafted from 7e50d52dc65227f494c6ad965917364fc08f0c5e)

--HG--
branch : 6.0

trytond/trytond/model/modelsql.py

@@ -144,7 +144,7 @@ class Exclude(Constraint):
             for column, operator in self.excludes)
         where = ''
         if self.where:
-            where = ' WHERE ' + str(self.where)
+            where = ' WHERE (' + str(self.where) + ')'
         return 'EXCLUDE (%s)' % exclude + where
 
     @property