Python3 compat changes to ssh agent support especially for winows

Hi,

Not sure if you're accepting pull-requests - feel free to ignore if not.

I was using your python3 port of paramiko and hit some issues when using it with pageant acting as the ssh-agent.
Fixes are hopefully quite straight forward.
The hardest to track down was the need for the __bytes__ function on the AgentKey class.
_parse_service_accept calls bytes(key) - probably used to be str - and without __bytes__ being implemented was getting garbage back.

Anyway, your python3 fork has been very useful and I thought I'd try and give a bit back,