[FIX] mass_mailing: access error when 'Retry'

A user without 'Administration/Settings' access rights cannot use the
'Retry' button on a mass-mailing.

This is because only this level of power is allowed to unlink
`mail.mail` records. However, this button is supposed to be used by
regular mass mailing users.

opw-744616

addons/mass_mailing/models/mass_mailing.py

@@ -517,7 +517,7 @@ class MassMailing(models.Model):
     def retry_failed_mail(self):
         failed_mails = self.env['mail.mail'].search([('mailing_id', 'in', self.ids), ('state', '=', 'exception')])
         failed_mails.mapped('statistics_ids').unlink()
-        failed_mails.unlink()
+        failed_mails.sudo().unlink()
         self.write({'state': 'in_queue'})
 
     #------------------------------------------------------