Hashsum

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* TBW

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* Fix the wheel generation: generated wheel are not universal,
they only work with Python 3.6 and higher.

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* Drop support for Python < 3.6.
* Switch to GitHub Actions for CI.
* Fix verification of checksums in text mode on Windows.
* Re-factorize and simplify the test code.
* Add buildsystem support (`pyproject.toml`)
* Switch to declarative setup configuration (`setup.cfg`).

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* hashsum.VERSION renamed into hashsum.__version__
* added test utility functions
* prefer lowercase names for hash functions
* improve robustness against unknown/unavailable hash functions in
threaded checksum computation
* new `--failfast` option added to the test CLI
* improved benchmark script
* man page updated
* improved compatibility with `pytest <https://pytest.org>`_

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* Fix compatibility with the windows operating system

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* Minor packaging fixes