Makes FakeRepository from Ch5 more like one from Ch2 - #355

It looks like FakeRepository in Chapter 5 introduced in 50bfa5c was left in original state, while in the meantime ABC was introduced into chapter 2 in 1795366, so here we make it match again.

As a side note, the alternative approach would be to use classmethod instead of staticmethod but it depends very much on personal preferences. It's not what I propose in that PR, but it could look like that:

class FakeRepository(repository.AbstractRepository):
    @classmethod
    def for_batch(cls, ref, sku, qty, eta=None):
        return cls([model.Batch(ref, sku, qty, eta)])