Solution to exclude base class from registry

base.py

        try:
            Spider
        except NameError:
            # Spider name is not known yet so we are either
            # initializing Spider class or we are initializing
            # some class that is not inherited from Spider
            # This mean that we do not need to add this class
            # to registry
            pass
        else:
            spider_registry.add_spider_class(cls)