Add support for type hints with mypy

[Kazhuu]

Configure mypy for this project and add some basic type hints to get started. If possible also run type checks as part of the Github Actions pipeline if needed. Cannot give direct points because not so familiar with this tool yet.

Outcomes:

• Typecheck is required part of the project, either locally run or as part of the pipeline.
• Add some types to see the type hints work.
• Add some documentation about this to the readme.