pymodbus version 1.5.2 did not support asyncio, and in general
the async handling have been improved a lot in version 2.3.0.

updated core/requirement*txt

committing result of 'python3 -m script.hassfest'.

change setup() --> async_setup and update() --> async_update()

Use async_setup_platform() to complete the async connection to core.

listen for EVENT_HOMEASSISTANT_START happens in async_setup()
so it needs to be async_listen.

But listen for EVENT_HOMEASSISTANT_STOP happens in start_modbus()
which is a sync. function so it continues to be listen().

setup of pymodbus is logically connected to the class modbushub,
therefore move it into the class.

Delay construction of pymodbus client until event
EVENT_HOMEASSISTANT_START arrives.

convert pymodbus calls to refer to the async library.

Remark: connect() is no longer needed, it is done when constructing
the client. There are also automatic reconnect.

Use async functions for read/write from pymodbus.

change thread.Lock() to asyncio.Lock()

When connecting, via Modbus-TCP, so some equipment (like the
huawei sun2000 inverter), they need time to prepare the protocol.

Solution is to add a asyncio.sleep(x) after the connect() and before
sending the first message.

Add optional parameter "delay" to Modbus configuration.
Default is 0, which means do not execute asyncio.sleep().

pylint does not accept that a class construction __new__
can return a tuple.

Create const.py with constants only used in
the modbus integration.

Duplicate entries are removed, but NOT any entry that would
lead to a configuration change.

Some entries were the same but with different names, in this
case renaming is done.

Also correct the tests.

Connection error handling depends on the hub, not the
entity, therefore it is logical to have the handling in
ModbusHub.

All pymodbus call are added to 2 generic functions (read/write)
in order not to duplicate the error handling code.

Added property "available" to signal if the hub is connected.

Solve CI problems.

close() no longer exist in the pymodbus library, use
del client instead.

Adjust code based on review comments.

Pymodbus in asyncio mode do not use twister but still throws a
warning if twister is not installed, this warning goes into
homeassistant.log and can thus cause confusion among users.

However installing twister just to avoid the warning is not
the best solution, therefore removing dependency on twister.

remove commented out code.