Ability to "pin" TLS version and/or ciphers

[rugk]

It would be nice if you could not only pin the cert hash, but also the ciphers and the TLS version to use. This would effectively make any downgrade attacks impossible and ensure that always the best encryption is used.

E.g. you can also do this in the PHP-SDK and there is an open issue for the Python SDK threema-ch/threema-msgapi-sdk-python#13.