gh-122728: Fix SystemError in PyEval_GetLocals() (#122735)

Fix PyEval_GetLocals() to avoid SystemError ("bad argument to
internal function"). Don't redefine the 'ret' variable in the if
block.

Add an unit test on PyEval_GetLocals().

Python/ceval.c

@@ -2499,7 +2499,7 @@ PyEval_GetLocals(void)
         PyFrameObject *f = _PyFrame_GetFrameObject(current_frame);
         PyObject *ret = f->f_locals_cache;
         if (ret == NULL) {
-            PyObject *ret = PyDict_New();
+            ret = PyDict_New();
             if (ret == NULL) {
                 Py_DECREF(locals);
                 return NULL;