bpo-1635741: Fix compiler warning in _stat.c (GH-19822)

Cast Py_ARRAY_LENGTH() size_t to int explicitly.
diff --git a/Modules/_stat.c b/Modules/_stat.c
index 45a4e08..c7090c0 100644
--- a/Modules/_stat.c
+++ b/Modules/_stat.c
@@ -563,7 +563,7 @@
         "ST_CTIME"
     };
 
-    for (int i = 0; i < Py_ARRAY_LENGTH(st_constants); i++) {
+    for (int i = 0; i < (int)Py_ARRAY_LENGTH(st_constants); i++) {
         if (PyModule_AddIntConstant(module, st_constants[i], i) < 0) {
             return -1;
         }