Missing type annotations

[mjdemilliano]

High quality Python code benefits from type hints.

This enables discovering developer errors by static analysis instead of relying only on runtime tests. It also helps spotting API-breaking changes.

This would require adding type hints to the Python code. At the very least the public functions should be annotated.

Additionally it would be recommended to add mypy to tox and the CI/CD workflow to make sure that everything remains working when future changes are made.