Add mypy for type checking

[Majsvaffla]

There are some type annotations already but there are some errors here and there. Since I think type annotations are very nice and helps a lot in both developing and using a library, I think the time has come to also use mypy for type checking.

The final goal here is to also type the json/dicts returned from the BankID API using typing.TypedDict but that's another story.

I intentionally didn't add type annotations to bankid/experimental since it's...experimental anyway 😸

[hbldh]

I think this is a good thing to do. I would never have done it myself, but I see the value.

Could you please add a contributing.rst in the docs or something similar and provide me (and potential other contributors) information about how to conform to this?

[Majsvaffla]

Could you please add a contributing.rst in the docs or something similar and provide me (and potential other contributors) information about how to conform to this?

Absolutely, good idea!

[hbldh]

This looks amazing! Great work. I will merge this now and release it soon.