🩹(backend) address linter flakiness on Email tests

Pylint was randomly failing due to a warning while unpacking emails.
The W0632 (Possible unbalanced tuple unpacking) was triggered.

Replace tuple unpacking by an explicitly accessing the first element of
the array using index.

src/backend/core/tests/test_models_invitations.py

@@ -257,7 +257,7 @@ def test_models_team_invitations_email():
     assert len(mail.outbox) == 1
 
     # pylint: disable-next=no-member
-    (email,) = mail.outbox
+    email = mail.outbox[0]
 
     assert email.to == [invitation.email]
     assert email.subject == "Invitation to join Desk!"