Use the new Py_ARRAY_LENGTH macro
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 2e36c73..f1eac91 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -1593,7 +1593,7 @@
              {-0xfffffffL, 28, -1}};
     int i;
 
-    for (i = 0; i < sizeof(testcases) / sizeof(struct triple); ++i) {
+    for (i = 0; i < Py_ARRAY_LENGTH(testcases); ++i) {
         PyObject *plong = PyLong_FromLong(testcases[i].input);
         size_t nbits = _PyLong_NumBits(plong);
         int sign = _PyLong_Sign(plong);