Patch signals to use post_delete, instead of pre_delete

From now on the signal will only get executed after the Role has been deleted
The commit also introduces minor changes in the tests of roles

Author: D0rs4n

pydis_site/apps/api/signals.py

@@ -1,10 +1,10 @@
-from django.db.models.signals import pre_delete
+from django.db.models.signals import post_delete
 from django.dispatch import receiver
 
 from pydis_site.apps.api.models.bot import Role, User
 
 
-@receiver(signal=pre_delete, sender=Role)
+@receiver(signal=post_delete, sender=Role)
 def delete_role_from_user(sender: Role, instance: Role, **kwargs) -> None:
     """Unassigns the Role (instance) that is being deleted from every user that has it."""
     for user in User.objects.filter(roles__contains=[instance.id]):

pydis_site/apps/api/tests/test_roles.py

@@ -4,7 +4,6 @@
 from ..models import Role, User
 
 
-
 class CreationTests(AuthenticatedAPITestCase):
     @classmethod
     def setUpTestData(cls):
@@ -96,11 +95,11 @@ def test_role_list(self):
         url = reverse('api:bot:role-list')
 
         response = self.client.get(url)
-        self.assertContains(response, text="id", count=4, status_code=200)
+        self.assertContains(response, text="id", count=5, status_code=200)
 
         roles = response.json()
         self.assertIsInstance(roles, list)
-        self.assertEqual(len(roles), 4)
+        self.assertEqual(len(roles), 5)
 
         for role in roles:
             self._validate_roledict(role)