[WIP] Python 3 Support

[0x6d6f7468]

Hello! Here's a PR to add Python 3 support to SIET.

Marked WIP because I still want to test some of the more invasive features to make sure everything works. The TFTP server logic and all syntax has been made to support both Python 2 and 3. I think the hex encode/decode logic can be cleaned up even further because the Python 3 method is backwards compatible with Python 2.

Most of what I've been able to test so far has been config file retrieval. I have a Cisco device to experiment with but I don't think it supports SMI.

Happy to answer any questions there may be. In the future, we might want to consider dropping Python 2 support entirely but I think that's a way off yet.