Fix readline.c compiler warning. (GH-98738)

```
Modules/readline.c:1260:37: warning: assigning to 'char *' from 'const char *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
    completer_word_break_characters =
                                    ^
```

Modules/readline.c

@@ -1258,9 +1258,9 @@ setup_readline(readlinestate *mod_state)
     rl_attempted_completion_function = flex_complete;
     /* Set Python word break characters */
     completer_word_break_characters =
-        rl_completer_word_break_characters =
         strdup(" \t\n`~!@#$%^&*()-=+[{]}\\|;:'\",<>/?");
         /* All nonalphanums except '.' */
+    rl_completer_word_break_characters = completer_word_break_characters;
 
     mod_state->begidx = PyLong_FromLong(0L);
     mod_state->endidx = PyLong_FromLong(0L);