accept Signature in verify_signature()

[jku]

A signature object was added into SSLib but verify_signature() wasn't updated to accept it.
It leads to weird code:

if not sslib_keys.verify_signature(
    sslib_key,
    signature.to_dict(), # <- why am I serializing the signature here ?
    data,
):
    print ("oh no")

[lukpueh]

I think we can change this into a "refactor internals of new signer API"-issue. cc @jku

[jku]

Yes I think I'm also fine with closing this... If the signer module provides me with everything I as a user need, I don't really care how the keys module operates.