Issue #8715: Create PyUnicode_EncodeFSDefault() function: Encode a Unicode

object to Py_FileSystemDefaultEncoding with the "surrogateescape" error
handler, return a bytes object. If Py_FileSystemDefaultEncoding is not set,
fall back to UTF-8.

Modules/_tkinter.c

@@ -3147,9 +3147,7 @@ PyInit__tkinter(void)
        it also helps Tcl find its encodings. */
     uexe = PyUnicode_FromWideChar(Py_GetProgramName(), -1);
     if (uexe) {
-        cexe = PyUnicode_AsEncodedString(uexe,
-                                         Py_FileSystemDefaultEncoding,
-                                         NULL);
+        cexe = PyUnicode_EncodeFSDefault(uexe);
         if (cexe)
             Tcl_FindExecutable(PyBytes_AsString(cexe));
         Py_XDECREF(cexe);