Add support for pymodbus 3.x.x

[eode]

This is an implementation of the changes mentioned in #35, plus the removal of the from pymodbus.compat import iteritems line, as pymodbus.compat doesn't exist in version 3, and the line was unused anyways.

Summary of changes for classic_mqtt.py:

• Catch import error if pymodbus 3 doesn't load, and try pymodbus 2

Summary of changes for support/classic_modbusdecoder.py:

• Catch import error if pymodbus 3 doesn't load, and try pymodbus 2
• Flag what pymodbus version we're using
• use slave= for version 3 and unit= for version 2.

I could also just drop support for pymodbus 2, but it seems pretty easy and straightforward to keep it.

[eode]

ping?

[eode]

Thanks.