mail.notification has been removed. The notify is done directly on the res.partner and channel

mail_optional_follower_notification/models/__init__.py

@@ -3,4 +3,4 @@
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
 
 from . import mail_message
-from . import mail_notification
+from . import res_partner

mail_optional_follower_notification/models/mail_notification.py

@@ -1,19 +0,0 @@
-# -*- coding: utf-8 -*-
-# Copyright 2016 ACSONE SA/NV (<http://acsone.eu>)
-# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
-
-from openerp import models, api
-
-
-class MailNotification(models.Model):
-    _inherit = 'mail.notification'
-
-    @api.model
-    def _notify(self, message_id, partners_to_notify=None,
-                force_send=False, user_signature=True):
-        if self.env.context.get('force_partners_to_notify'):
-            partners_to_notify =\
-                self.env.context.get('force_partners_to_notify')
-        super(MailNotification, self)._notify(
-            message_id, partners_to_notify=partners_to_notify,
-            force_send=force_send, user_signature=user_signature)

mail_optional_follower_notification/models/res_partner.py

@@ -0,0 +1,18 @@
+# -*- coding: utf-8 -*-
+# Copyright 2016 ACSONE SA/NV (<http://acsone.eu>)
+# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
+
+from openerp import models, api
+
+
+class ResPartner(models.Model):
+    _inherit = 'res.partner'
+
+    @api.multi
+    def _notify(self, message, force_send=False, user_signature=True):
+        if self.env.context.get('force_partners_to_notify'):
+            partners_to_notify =\
+                self.env.context.get('force_partners_to_notify')
+            self = self.filtered(lambda p: p.id in partners_to_notify)
+        super(ResPartner, self)._notify(
+            message, force_send=False, user_signature=True)