Decide what to do about IDNA encoding

[mithrandi]

Cryptography no longer IDNA encodes names for us, should we be doing this instead?

[markrwilliams]

Cryptography no longer IDNA encodes names for us, should we be doing this instead?

Yes - perhaps this can begin by making ICertificateStore impose an encoding for server_name? This should be pretty easy for DirectoryStore because it's already got to deal with whatever sys.getfilesystemencoding claims the PEM path's encoding should be.