Change forkserver test to only run below 3.13.8

test/test_multiprocessing_spawn.py

@@ -265,6 +265,12 @@ class ParallelForkServerShouldWorkTest(TestCase, _TestMultiProcessing):
 )
 class ParallelForkServerPerfTest(TestCase):
 
+    @unittest.skipIf(
+        sys.version_info >= (3, 13, 8),
+        "Python 3.13.8+ changed forkserver module caching behavior",
+        # https://docs.python.org/3.13/whatsnew/changelog.html
+        # gh-126631
+    )
     def test_forkserver_perf(self):
 
         start_method = 'forkserver'