On ResourceWarning, log traceback where the object was allocated

Issue #26567:

* Add a new function PyErr_ResourceWarning() function to pass the destroyed
  object
* Add a source attribute to warnings.WarningMessage
* Add warnings._showwarnmsg() which uses tracemalloc to get the traceback where
  source object was allocated.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 65b20be..3f22d14 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -12111,8 +12111,8 @@
          */
         ++Py_REFCNT(iterator);
         PyErr_Fetch(&exc, &val, &tb);
-        if (PyErr_WarnFormat(PyExc_ResourceWarning, 1,
-                             "unclosed scandir iterator %R", iterator)) {
+        if (PyErr_ResourceWarning((PyObject *)iterator, 1,
+                                  "unclosed scandir iterator %R", iterator)) {
             /* Spurious errors can appear at shutdown */
             if (PyErr_ExceptionMatches(PyExc_Warning))
                 PyErr_WriteUnraisable((PyObject *) iterator);