Update IntegrityError pattern for PostgreSQL 13.

A more specific error is raised on higher PostgreSQL versions, for example, as
with the PostgreSQL version running on my local machine. This commit fixes the
error message pattern matching behaviour by adding an optional match to the
detailed error message.

pydis_site/apps/api/tests/test_infractions.py

@@ -664,7 +664,10 @@ def test_unique_constraint_accepts_active_infractions_for_different_users(self):
         )
 
     def test_integrity_error_if_missing_active_field(self):
-        pattern = 'null value in column "active" violates not-null constraint'
+        pattern = (
+            'null value in column "active" (of relation "api_infraction" )?'
+            'violates not-null constraint'
+        )
         with self.assertRaisesRegex(IntegrityError, pattern):
             Infraction.objects.create(
                 user=self.user,