Set a callback that will be called when a server name is provided by the SSL/TLS client in the SNI extension.
If the argument is None then the callback is disabled. The method
is called with the SSLSocket, the server name as a string, and the
SSLContext object.
See RFC 6066 for details of the SNI extension.
3.15: After the callback assigns a new SSLSocket.context, later ClientHello messages on the connection are dispatched to the new context’s sni_callback.