Minimalmodbus

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* Added a .readthedocs.yaml configuration file.

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New features:

* Support for 64-bit integers in read_long() and write_long().
* Allow an external serial port object to be used in the Instrument constructor.

Other fixes:

* Change internal data representation to Python bytes objects (from bytestrings).
* Allow integers in write_float(). Fixes github 78.
* Adapt style to linting tools.
* Improve Github Action script.
* Improve scripts for tests on the Delta DTB4824 instrument.
* Moved the contents of setup.cfg to pyproject.toml.
* Documentation updates, for example on building with Yocto.

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* Improved documentation
* Finetuned Github Actions workflow

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New features:

* Type hints available
* Support for broadcast (slave address 0), by Jan Breuer.
* Measurement of round-trip time

Breaking changes:

* Dropping support for Python 2

Other fixes:

* Starting to convert internal data representation as bytes (it had to be strings
when supporting Python 2)
* Converted from setup.py to setup.cfg
* Use Github Actions instead of Travis CI

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* Adjusted settings for hardware tests
* Improved developer documentation

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* Corrected version number