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

diff --git a/Lib/test/_testcppext.cpp b/Lib/test/_testcppext.cpp
index 0e381a7..82b4713 100644
--- a/Lib/test/_testcppext.cpp
+++ b/Lib/test/_testcppext.cpp
@@ -86,7 +86,7 @@
     // 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);