Correct mypy type checking through PEP 561

[DemyCode]

Describe the feature you'd like
Correct mypy type checking through PEP 561

How would this feature be used? Please describe.
Will enable mypy type checking to use the type hintings of this repo.
This makes the usage of the repo much much easier for new comers and to avoid simple bugs.

#5027

[mollyheamazon]

PR merged. Thanks for the contribution!