GSS-API / SSPI authenticated Diffie-Hellman Key Exchange and user authentication with Python 3 support

Add Python 3 support for the GSS-API / SSPI authenticated Diffie-Hellman
Key Exchange and user authentication.
This patch supersedes pull request #250.
I agree that dh-group14-sha1 should be the prefered kex. We should also consider implementing dh-group14-sha256 and dh-gex-sha256, because sha1 is considered insecure too.