gh-123504: Fix regression in _tkinter initializer (#123662)

* Add module traverse function to _tkinter.
* Set m_size to -1 (instead of 0).

Author: ZeroIntensity

Modules/_tkinter.c

@@ -3390,7 +3390,7 @@ DisableEventHook(void)
 }
 
 static int
-module_clear(PyObject *mod)
+module_clear(PyObject *Py_UNUSED(mod))
 {
     Py_CLEAR(Tkinter_TclError);
     Py_CLEAR(Tkapp_Type);
@@ -3399,16 +3399,28 @@ module_clear(PyObject *mod)
     return 0;
 }
 
+static int
+module_traverse(PyObject *Py_UNUSED(module), visitproc visit, void *arg)
+{
+    Py_VISIT(Tkinter_TclError);
+    Py_VISIT(Tkapp_Type);
+    Py_VISIT(Tktt_Type);
+    Py_VISIT(PyTclObject_Type);
+    return 0;
+}
+
 static void
 module_free(void *mod)
 {
-    module_clear((PyObject *)mod);
+    (void)module_clear((PyObject *)mod);
 }
 
 static struct PyModuleDef _tkintermodule = {
     PyModuleDef_HEAD_INIT,
     .m_name = "_tkinter",
+    .m_size = -1,
     .m_methods = moduleMethods,
+    .m_traverse = module_traverse,
     .m_clear = module_clear,
     .m_free = module_free
 };