In https://github.com/lelit/cpython/commit/3e5e557a876831a99c21f5a173623cb05ff48abf
I reimplemented the functionality in a slightly different and hopefully better
way, rebasing it on current master.

IANAL, but I think that the new approach is different enough from the original
GNU bash code to be considered safe from the license point of view.

I still could not test it against the editline alternative implementation:
AFAICT all the functions and symbols I used are exposed by that library too,
so it may work without resorting to #ifdefs.

In that regards however, I think we could and should take a different approach
in determining which underlying implementation is used: GNU readline >= 4.1
exposes a rl_gnu_readline_p flag that could be used at configure time to
define a IS_GNU_READLINE, and then rely on that to implement different code
paths within the readline.c module.

Please let me know if I should upload a traditional patch, instead of
referencing my branch on github.

As usual, thank you for any review and feedback!