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/timemodule.c b/Modules/timemodule.c
index 1a85662..5e6cd6c 100644
--- a/Modules/timemodule.c
+++ b/Modules/timemodule.c
@@ -502,7 +502,7 @@
         buf.tm_isdst = 1;
 
 #ifdef HAVE_WCSFTIME
-    format = PyUnicode_AsWideCharString((PyUnicodeObject*)format_arg, NULL);
+    format = PyUnicode_AsWideCharString(format_arg, NULL);
     if (format == NULL)
         return NULL;
     fmt = format;