Adding Type Hints

[daikikatsuragawa]

I propose to add "Type Hints" to the code written in Python, which is not in this project. The benefits are as follows.

• Improved code readability
• Improved code robustness (but also requires implementation of other tools such as mypy)

[amit-sharma]

That will be really useful, @daikikatsuragawa You can go ahead and raise a PR for this. mypy is the standard tool to do this, so including mypy (perhaps in the dev/test dependencies) sounds good.

[daikikatsuragawa]

@amit-sharma Thanks! I will proceed with the plan.