add typing support to make and prepare

[stohrendorf]

This adds proper typing for enabling at least PyCharm to support proper code completion and silence type warnings in baker.make and baker.prepare calls.

1. Before, PyCharm was always complaining about the first parameter to baker.make not being of Type[ModelBase], which is correct, as we're passing derived models, not the ModelBase.
2. Before, PyCharm was unable to provide code completion for the resulting instance. Although this is still not perfect because of the _quantity parameter, it now correctly lists fields and methods for the passed model class besides the list interface. I.e., we do not need to write mocked_model: SomeModel = baker.make(SomeModel) every time, and instead can rely on automatic type deduction now without explicitly specifying the model type of the variable.

[timjklein36]

Looks like the "base" class is not exactly correct (see inline comment). Otherwise the type union for the return looks fine (replacing the TypeVar with the correct base type).

[berinhard]

Thanks for your PR @stohrendorf and @timjklein36 for the initial review!

Happy baking 🍰