Fixed #31051 -- Allowed dumpdata to handle circular references in natural keys.

Since #26291 forward references in natural keys are properly handled by
loaddata, so sorting depenencies in dumpdata doesn't need to break on
cycles. This patch allows circular references in natural keys by
breaking sort_depenencies() on loops.

tests/fixtures/tests.py

@@ -880,3 +880,23 @@ class CircularReferenceTests(DumpDataAssertMixin, TestCase):
             '{"model": "fixtures.circularb", "pk": 1, '
             '"fields": {"key": "y", "obj": 1}}]',
         )
+
+    def test_circular_reference_natural_key(self):
+        management.call_command(
+            'loaddata',
+            'circular_reference_natural_key.json',
+            verbosity=0,
+        )
+        obj_a = CircularA.objects.get()
+        obj_b = CircularB.objects.get()
+        self.assertEqual(obj_a.obj, obj_b)
+        self.assertEqual(obj_b.obj, obj_a)
+        self._dumpdata_assert(
+            ['fixtures'],
+            '[{"model": "fixtures.circulara", '
+            '"fields": {"key": "x", "obj": ["y"]}}, '
+            '{"model": "fixtures.circularb", '
+            '"fields": {"key": "y", "obj": ["x"]}}]',
+            natural_primary_keys=True,
+            natural_foreign_keys=True,
+        )