Improve folder structure for pytests

[amarvin]

The folder structure here isn't best practices. It may hinder test discovery or execution either by IDE or from terminal (e.g. VS Code can find an run the tests, but running pytest in terminal gives ModuleNotFoundError).

• https://github.com/ferdinandvanwyk/python_project_example
• https://docs.pytest.org/en/latest/explanation/goodpractices.html
• https://docs.pytest.org/en/latest/reference/customize.html#pyproject-toml