[feat] set up testing framework and code formatting tools

[choinek]

Add testing infrastructure (pytest) and code quality tools (black, isort) to the project, including an example unit test.

• Requirements_dev.txt
• Add pytest for testing
• Add black for code formatting
• Add isort for import sorting
• Set up dev configuration in pyproject.toml
• Add a sample unit test

[pkarw]

That's cool. But if it won't get in in the current PR - that's also fine, we can add it incrementally in the future no problem