Add Python 3 support

[jonyesno]

This MR:

• encodes/decodes strings to/from bytes when talking to the HAProxy socket
• tweaks the try … except syntax

both of which make the plugin work under Python 3.

This addresses the following logged exceptions:

Unhandled python exception in importing module: SyntaxError: invalid syntax
Unhandled python exception in read callback: TypeError: a bytes-like object is required, not 'str'
Unhandled python exception in read callback: TypeError: must be str, not bytes

Disclaimer: my Python skills are very limited, but this fixed it for me.