Mark _Py_char2wchar() input argument as constant

Include/fileutils.h

@@ -6,7 +6,7 @@ extern "C" {
 #endif
 
 PyAPI_FUNC(wchar_t *) _Py_char2wchar(
-    char *arg);
+    const char *arg);
 
 PyAPI_FUNC(char*) _Py_wchar2char(
     const wchar_t *text);

Python/fileutils.c

@@ -17,7 +17,7 @@
    PyMem_Free() to free the memory), or NULL on error (conversion error or
    memory error). */
 wchar_t*
-_Py_char2wchar(char* arg)
+_Py_char2wchar(const char* arg)
 {
     wchar_t *res;
 #ifdef HAVE_BROKEN_MBSTOWCS