* add pyserial to manifest
pymodbus is very developer oriented and assumes every developer
adapt the requierements.txt to his/hers needs.
Our requirements.txt is different it contains all posibilities allowing
user to later change configuration without having to install extra
packages.
As a consequence manifest.json needs to include the pyserial.
* modbus: make truly async client creation
Make hass call listen_once async.
Integrate content of start_modbus into async_setup.
Do not use the boiler plate create tcp client function from
pymodbus as it is sync, and also does not work well with asyncio,
instead call the init_<type> directly, since that is async.
* both component/modbus and component/serial uses pyserial-async
but with slighty different version requirements.
Combined the 2.
* Review 1
* Review 2
* Review
@staticmethod is no good, because the function uses class variables.
* Review
Pytest is sometimes a bit cryptic, lets hope this does it.
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