I think it would be good to add that the default encoding changes from platform-dependent to UTF-8, especially since it might break on some platforms where UTF-8 isn't the default. What do you think?

Could use locale.getpreferredencoding() as default value. This will keep behavior stay same as before, because it's default encoding when we call open.

You can simply pass encoding=None to let io.open infer the user’s default environment encoding.

Passing encoding=None sounds like the way to go, breaking the existing API doesn't sound like a good idea.

I've updated the encoding to be platform independent now, it's not a breaking change now. cc: @bbc2