This patch is functionally equivalent, but advertises 'tls-unique' support in a bit different way.

HAS_TLS_UNIQUE is not exposed in the python 'ssl' module, instead a list 'CHANNEL_BINDING_TYPES' is provided (empty when 'tls-unique' is not supported). get_channel_binding raises ValueError if the argument is not on this list. This way the API can be extended to other channel binding types without adding new constants or functions. Adding a new channel binding type would not need any modifications in the API client code (if it is designed to use arbitrary cb types).