Exclude an always true branch from coverage in comments' subscription view method

The form is composed of two checkbox which are either absent for False
or present with any value for True. It's not possible for it to be not
valid.

The test is kept here for future needs.

debexpo/comments/views.py

@@ -79,7 +79,12 @@ def subscribe(request, name):
         form = SubscriptionForm(request.POST, instance=instance)
         package = request.POST.get('next')
 
-        if form.is_valid():
+        # The condition here is just for show. The form is composed of two
+        # checkbox which are either absent for False or present with any value
+        # for True. It's not possible for it to be not valid.
+        #
+        # The test is kept here for future needs.
+        if form.is_valid():  # pragma: no branch
             subscription = form.save(commit=False)
             subscription.user = request.user
             subscription.package = name