[FIX] website_mail_channel: prevent crash in python3 email lib

`AttributeError: 'tuple' object has no attribute 'startswith'`

When given headers that are tuples. They are expected to be string, as per [1].
The issue was introduced with [2], indeed the code was moved from inside a dict
where `,` was the usual dict separator to outside of a dict where `,` at the end
made them tuples.

To reproduce the issue, run the `test_mail` suite after having
`website_mail_channel` installed.

Issue highlighted as part of task-2178641

[1] https://docs.python.org/3/library/email.policy.html#email.policy.Policy.header_store_parse
[2] cae1c3977f

closes odoo/odoo#43698

X-original-commit: 7e5a7245e86d8a3bcdab83c740cb4d188d009a40
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/website_mail_channel/models/mail_channel.py

@@ -5,8 +5,7 @@ import hmac
 
 from werkzeug import urls
 
-from odoo import api, models
-from odoo.tools.safe_eval import safe_eval
+from odoo import models
 from odoo.addons.http_routing.models.ir_http import slug
 
 
@@ -16,9 +15,9 @@ class MailGroup(models.Model):
     def _notify_email_header_dict(self):
         headers = super(MailGroup, self)._notify_email_header_dict()
         base_url = self.env['ir.config_parameter'].sudo().get_param('web.base.url')
-        headers['List-Archive'] = '<%s/groups/%s>' % (base_url, slug(self)),
-        headers['List-Subscribe'] = '<%s/groups>' % (base_url),
-        headers['List-Unsubscribe'] = '<%s/groups?unsubscribe>' % (base_url,),
+        headers['List-Archive'] = '<%s/groups/%s>' % (base_url, slug(self))
+        headers['List-Subscribe'] = '<%s/groups>' % (base_url)
+        headers['List-Unsubscribe'] = '<%s/groups?unsubscribe>' % (base_url,)
         return headers
 
     def _send_confirmation_email(self, partner_ids, unsubscribe=False):