Yaspin

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* Migrate to ``poetry`` for dependencies management, building and publishing project
* Add tests for Python 3.8
* Deprecate support for Python 3.4
* Run tests under Ubuntu 18.04
* Update dev dependencies to the most recent ones (compatible with Python 2.7)
* Remove Tox from the project (use CI for tests under different versions of Python)

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* Allow use inside zip bundled package
* Code improvements

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* Update cli-spinners to v2.2.0

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* fix(29): race condition between spinner thread and ``write()``

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* fix: remove extra ``\b`` written to stdout. Fixes ``write()`` in rxvt terminal

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* fix(26): traceback on PYTHONOPTIMIZE=2