Use pymodbus simulator instead of python mock

[alexrudd2]

Closes #1.

Use a pymodbus server (with a datastore for all modbus registers) to simulate the full Modbus request/reply roundtrip. This is more robust than mock.py.

Note the intermediate commit of using the full pymodbus simulator with HTTP functionality. It's not needed but might as well keep it in the commit log for future info.

Known issues:

• The teardown for the simulated server sometimes happens multiple times / out-of-order with pytest-xdist on pymodbus 2.5.3

[alexrudd2]

The failing test is an actual failure on pymodbus 3.7.x. (I've got it pinned in pyproject.toml as a stop-gap)