Type annotations about smart_importer

[johannesjh]

Background: I started adding type annotations because I liked them better than comments. Not sure how they can be used in python to verify correctly typed data.

I am honestly not sure about how to follow up on this. E.g.,

• We could introduce static type checks, either while running the program or during continuous integration. For example, after quick googling, I found mypy, which people are using to perform type checking.
• I found that some articles argued against static type checks in favor of duck typing. Sure, we could introduce ducky type checks, but they would probably add many lines of code, making refactorings more difficult, without a lot of benefit.
• Or else we could drop the type declarations altogether and simply throw runtime errors if data does not match the program's expectations. But I like type declarations for documentation purposes.

[yagebu]

Afaict setting up static type checking is not trivial since it needs to know the types of structures from the dependencies too. However, if we want any kind of type checking, static is the way to go.

If they're just for documentation purposes, I'd be for having fewer of them and in particular not having them in places where it's absolutely obvious (like transaction being, well, a list of Transaction objects).