Tasks3

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* Flatten tasks3.db.model module into models.py
* Linting changes
* Minor refactoring

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* Remove pytest from dependency and let tox handle testing.

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* Migrate testing to github-workflow
* Update SQLAlchemy package version.
* Switch deployment workflow to python 3.9

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* Drop support for python<=3.8

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* Implement tasks3.add
* Implement tasks3.edit
* Implement tasks3.remove

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* Add support for a yaml configuration file.
* Add database to store Tasks, db models and api to interact with db.
* Switch to using requirements.txt for managing dependency and add
back the support for py35.
* Add a bunch of type annotations.
* Update dependency:
* pip to 20.2
* pytest to 6.0.1
* tox to 3.18.1
* coverage to 5.2.1