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 =
                                    ^
```
(cherry picked from commit 29b391b1378577825a658b14764a8ff3e0b5c958)

Co-authored-by: Benjamin Peterson <benjamin@python.org>