Tasks3

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* Switch docs theme to ``sphinx_rtd_theme``.

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* Add workflow to check for package compatability with PyPI.
This should make sure that the issue with v0.3.0 does not occur again.

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* Fix README to render on PyPI.

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* Remove ``tasks3 db init`` cli command.
* Implement ``tasks3 task add`` cli command.
* Implement ``task.yaml``, ``task.short``, ``task.one_line`` methods to display task.

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* Use dataclass to store configuration settings.
* Flatten tasks3.config module into config.py file.

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* Remove usage of deprecated SQLAlchemy api ``db_engine.table_names``.
* Remove deprecated pytest configuration option ``collect_ignore``.