Looks like it was always getting lucky in the past, as sys.path requires strings, and the "path" argument here is an alternative to it. The cache was definitely not working as intended.

So while it's not clearly documented anywhere (other than the related pkgutil.extend_path() method saying it'll ignore non-str paths), we do need to only be passing str into _importlib_bootstrap.

I can't do the pkgutil updates right now, but I think it's just get_importer than needs an os.fsdecode() call around its argument. Maybe some others in the same module. And add tests :)