closes gh-106479: fix typo in __cplusplus macro (gh-106480)

Author: dtrodrigues

Lib/test/_testcppext.cpp

@@ -86,7 +86,7 @@ test_api_casts(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
     // gh-93442: Pass 0 as NULL for PyObject*
     Py_XINCREF(0);
     Py_XDECREF(0);
-#if _cplusplus >= 201103
+#if __cplusplus >= 201103
     // Test nullptr passed as PyObject*
     Py_XINCREF(nullptr);
     Py_XDECREF(nullptr);