Don't raise an exception on normal return from generator. (GH-19473)

Misc/NEWS.d/next/Core and Builtins/2020-04-11-13-07-49.bpo-4022.Ctpn_F.rst

@@ -0,0 +1 @@
+Improve performance of generators by not raising internal StopIteration.

Objects/genobject.c

@@ -231,7 +231,8 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc, int closing)
             if (PyAsyncGen_CheckExact(gen)) {
                 PyErr_SetNone(PyExc_StopAsyncIteration);
             }
-            else {
+            else if (arg) {
+                /* Set exception if not called by gen_iternext() */
                 PyErr_SetNone(PyExc_StopIteration);
             }
         }