bpo-39378: partial of PickleState struct should be traversed. (GH-18046)

(cherry picked from commit 1f577ce363121d590b51abf5c41d1bcf3d751436)

Co-authored-by: Hai Shi <shihai1992@gmail.com>
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
index 55e2734..55affb2c 100644
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -7961,6 +7961,7 @@
     Py_VISIT(st->import_mapping_3to2);
     Py_VISIT(st->codecs_encode);
     Py_VISIT(st->getattr);
+    Py_VISIT(st->partial);
     return 0;
 }