Introducing CI

[fao89]

You can see the CI here: https://github.com/fabricio-aguiar/tigre/runs/476998688?check_suite_focus=true

Reference: https://medium.com/@vanflymen/blazing-fast-ci-with-github-actions-poetry-black-and-pytest-9e74299dd4a5

Notes:

• I had to add some types because of mypy, I'm not sure if I declared the right types
• I made the CI only run on python 3.8, because it was specified on pyproject.toml