PyUnicode_AsWideCharString() takes a PyObject*, not a PyUnicodeObject*

All unicode functions uses PyObject* except PyUnicode_AsWideChar(). Fix the
prototype for the new function PyUnicode_AsWideCharString().
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 912ba17..473d805 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -1426,7 +1426,7 @@
     if (!PyArg_ParseTuple(args, "U", &unicode))
         return NULL;
 
-    buffer = PyUnicode_AsWideCharString((PyUnicodeObject*)unicode, &size);
+    buffer = PyUnicode_AsWideCharString(unicode, &size);
     if (buffer == NULL)
         return NULL;