Change typing of batch_load_fn to support subclass overriding

[markedwards]

Using a Protocol to type batch_load_fn does not play well with the documented pattern of overriding the method in a subclass.

For instance:

class TestDataLoader(DataLoader[str, str]):

    async def batch_load_fn(self, keys: list[str]) -> list[str]:
        return [key for key in keys]

This causes mypy to complain Signature of "batch_load_fn" incompatible with supertype "DataLoader".

Fixing by typing batch_load_fn as Callable[[List[KeyT]], Coroutine[Any, Any, List[ReturnT]]], which also avoids the conditional import of Protocol.

[markedwards]

@aawilson, do you have any concerns with this adjustment?

[aawilson]

Should be fine, I mentioned in the other thread I was surprised at the behavior because we use it (subclassed) with the protocol at work and the linter doesn't mind, I didn't poke at it harder but at the outset want to chalk it up to different linter behavior.