Mbc1Cartridge.__getitem__ will return None for RAM addresses when ram_enable is off.

Is this intended behavior? It seems to me like a more obvious default action would be to return 0x0, and I haven't checked, but that seems more like what hardware would do if you accessed a disabled thing.

Unless that action is supposed to signal an interrupt or something and None is a sigil return value to tell the emulator to trigger that interrupt, but if that is the case, then None should probably be returned explicitly, and a comment should be added explaining why, when all the other functions return integers.