So I'm failing to find any documentation for libedit, but it looks as though this error occurs if rl_read_init_file fails to find an .editrc file in the appropriate place.  If I create an empty .editrc file in my home directory, the error disappears.  (Having an .inputrc file doesn't seem to make a difference either way.)

Perhaps the solution is to ignore an OSError from readline.read_init_file in register_readline---i.e., add a try / except OSError there.  The other option would be to ignore a nonzero errno from rl_read_init_file, but that doesn't sound so good: I'd expect someone deliberately calling readline.read_init_file to get an exception if the initialization file isn't found.