Add type hinting to functions

[f0lie]

Sometimes it's hard to understand the intent of the code without type hints. It would help a lot in understand the the pieces of code that is added to classes.

It would likely take a while to migrate all functions to type hinting, but it's still worth discussing it in a bug.

[hiepdang-ml]

Agree! I found the book has great content but the code was poorly written by not following coding standards. No docstrings, no code comments, no type hints, poorly named variables. I hope they will rework on the code in later versions.