Use `pytest` to run our tests

[DiddiLeija]

Hi!

We have been working hard to automate many of our jobs (see this project board), and we're having great results.

We have a tests folder to work on running tests and linters. However, we don't have any test files on it, even when we want to use Pytest. And you can help us!

We will accept PRs to set up Pytest, create test files, etc. Please comment here if you are interested 👍.

[DiddiLeija]

I need to add a test for text_formatter.justify (see #59), but I'll accept tests for other pieces of the package 😉.