Pymodbus

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* Fix Issues with Serial client where in partial data was read when the response size is unknown.
* Fix Infinite sleep loop in RTU Framer.
* Add pygments as extra requirement for repl.
* Add support to modify modbus client attributes via repl.
* Update modbus repl documentation.
* More verbose logs for repl.

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* Fix unicode decoder error with BinaryPayloadDecoder in some platforms
* Avoid unnecessary import of deprecated modules with dependencies on twisted

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* Async client implementation based on Tornado, Twisted and asyncio with backward compatibility support for twisted client.
* Allow reusing existing[running] asyncio loop when creating async client based on asyncio.
* Allow reusing address for Modbus TCP sync server.
* Add support to install tornado as extra requirement while installing pymodbus.
* Support Pymodbus REPL
* Add support to python 3.7.
* Bug fix and enhancements in examples.
* Async client implementation based on Tornado, Twisted and asyncio

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* Fix serial client `is_socket_open` method

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* Fix device information selectors
* Fixed behaviour of the MEI device information command as a server when an invalid object_id is provided by an external client.
* Add support for repeated MEI device information Object IDs (client/server)
* Added support for encoding device information when it requires more than one PDU to pack.
* Added REPR statements for all syncchronous clients
* Added `isError` method to exceptions, Any response received can be tested for success before proceeding.
* Add examples for MEI read device information request

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* Improve transaction speeds for sync clients (RTU/ASCII), now retry on empty happens only when retry_on_empty kwarg is passed to client during intialization
* Fix tcp servers (sync/async) not processing requests with transaction id > 255
* Introduce new api to check if the received response is an error or not (response.isError())
* Move timing logic to framers so that irrespective of client, correct timing logics are followed.
* Move framers from transaction.py to respective modules
* Fix modbus payload builder and decoder
* Async servers can now have an option to defer `reactor.run()` when using `Start<Tcp/Serial/Udo>Server(...,defer_reactor_run=True)`
* Fix UDP client issue while handling MEI messages (ReadDeviceInformationRequest)
* Add expected response lengths for WriteMultipleCoilRequest and WriteMultipleRegisterRequest
* Fix _rtu_byte_count_pos for GetCommEventLogResponse
* Add support for repeated MEI device information Object IDs
* Fix struct errors while decoding stray response
* Modbus read retries works only when empty/no message is received
* Change test runner from nosetest to pytest
* Fix Misc examples