Implement PEP 393.
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 456a8a59..2e36c73 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -1355,7 +1355,7 @@
 test_Z_code(PyObject *self)
 {
     PyObject *tuple, *obj;
-    Py_UNICODE *value1, *value2;
+    const Py_UNICODE *value1, *value2;
     Py_ssize_t len1, len2;
 
     tuple = PyTuple_New(2);