Don't export pending_threadfunc from _testcapi. (GH-8075)

Modules/_testcapimodule.c

@@ -2429,7 +2429,8 @@ static int _pending_callback(void *arg)
 /* The following requests n callbacks to _pending_callback.  It can be
  * run from any python thread.
  */
-PyObject *pending_threadfunc(PyObject *self, PyObject *arg)
+static PyObject *
+pending_threadfunc(PyObject *self, PyObject *arg)
 {
     PyObject *callable;
     int r;