Do not lock table on SQLite when checking unsupported constraint

It is not needed as SQLite does not have an explicit lock mechanism but such
call restart the transaction anyway.
(grafted from 4cc5987a3b800121007968cebb5976a5cac2384b)

--HG--
branch : 7.0

trytond/trytond/model/modelsql.py

@@ -2135,7 +2135,8 @@ class ModelSQL(ModelStorage):
                 continue
             table = sql.table
             if isinstance(sql, (Unique, Exclude)):
-                lock(connection, cls._table)
+                if backend.name != 'sqlite':
+                    lock(connection, cls._table)
                 columns = list(sql.columns)
                 columns.insert(0, table.id)
                 in_max = transaction.database.IN_MAX // (len(columns) + 1)