Issue #21789: fix broken link (reported by Jan Varho)

Parser/tokenizer.c

@@ -534,7 +534,7 @@ decoding_fgets(char *s, int size, struct tok_state *tok)
             "Non-ASCII character '\\x%.2x' "
             "in file %.200s on line %i, "
             "but no encoding declared; "
-            "see http://www.python.org/peps/pep-0263.html for details",
+            "see http://python.org/dev/peps/pep-0263/ for details",
             badchar, tok->filename, tok->lineno + 1);
         PyErr_SetString(PyExc_SyntaxError, buf);
         return error_ret(tok);