Python 3 support

[jaapz]

We need to support python 3. This is going to need a complete rewrite of the modbus protocol implementation, as modbus_tk is not python3 compatible, and is generally not very nice to work with (bad exception handling, etc).

This issue serves as a central place for discussing python 3 support and implementation progress.

[OrangeTux]

A few notes about Modbus Tk:

• Lacks Python 3 support.
• Requests are handled in 1 thread.
• The server is started in a 1 thread, making exception handling and debugging hard.
• It doesn't implement all Modbus functions.