Generic itertools.chain (GH-19417)

diff --git a/Modules/itertoolsmodule.c b/Modules/itertoolsmodule.c
index e99c964..4ebeb74 100644
--- a/Modules/itertoolsmodule.c
+++ b/Modules/itertoolsmodule.c
@@ -2028,6 +2028,8 @@
      reduce_doc},
     {"__setstate__",    (PyCFunction)chain_setstate,    METH_O,
      setstate_doc},
+    {"__class_getitem__",    (PyCFunction)Py_GenericAlias,
+    METH_O|METH_CLASS,       PyDoc_STR("See PEP 585")},
     {NULL,              NULL}           /* sentinel */
 };