V3 finishes what I started in v2:

* Changed unchecked PyBytes_AsString() → PyBytes_AS_STRING()
* Testing more functions for non-ASCII characters

I tried to test it with Editline on Linux (using my patch for Issue 13501). There seem to be many quirks with my version of Editline, some of which are not easy to work around:

* Initial CR swallowed when entering non-ASCII
* set_completion_display_matches_hook(), set_pre_input_hook(), set_completer_delims() all do nothing useful
* get_history_item() not updated straight after read_history_file()

I suspect Apple has patched their version of Editline, but if these quirks exist on Apple as well, it might be simplest to skip the test for Editline.