Use the new ECDH interface from python-ecdsa

[tomato42]

https://github.com/warner/python-ecdsa/issues/128 is now fixed, so there is a clean interface for handling ECDH using python-ecdsa

use it instead of the current code for ECDH

we may need to do a small extension in python-ecdsa so that it doesn't accept raw ecdh, or compressed ecdh points when we didn't allow them