[FIX] base: make _MODES immutable

Author: mwath

odoo/addons/base/models/ir_rule.py

@@ -13,7 +13,7 @@ class IrRule(models.Model):
     _name = 'ir.rule'
     _description = 'Record Rule'
     _order = 'model_id DESC,id'
-    _MODES = ['read', 'write', 'create', 'unlink']
+    _MODES = ('read', 'write', 'create', 'unlink')
     _allow_sudo_commands = False
 
     name = fields.Char(index=True)