types: more general type hint for type_hooks

[sinback]

Update the type hint for type_hooks to accept the more general Mapping rather than Dict. It looks like transform_value will work with any type_hooks that supports __getitem__, which the Mapping ABC / type signature requires that users do.

This makes the typechecker happier with us since I want to start using a Mapping implementation for a type_hooks that we use, to allow a little more flexibility. I can't think of any particular drawbacks to having a more general type hint here.

Out of self-interest, I added a test which I think will ensure dacite continues to support the use case I have in mind (tests a Mapping which is exactly like a dict but has an augmented __getitem__ method).