Type of Python function pointer

Question

For Python 3, it turned out as a good practise for me, to hint data types for function parameters and return types. For example:

def icecream_factory(taste: str='Banana') -> Ice:
    ice = Ice(taste)
    ice.add_cream()
    return ice

This works perfectly for all simple data types and classes. But now I need to use this with a "function pointer":

class NotificationRegister:

    def __init__(self):
        self.__function_list = list()
        """:type: list[?????]"""

    def register(self, function_pointer: ?????) -> None:
        self.__function_list.append(function_pointer)

def callback():
    pass

notification_register = NotificationRegister()
notification_register.register(callback)

What must be put at the ????? to make clear that a function pointer is required here? I tried function, because type(callback) is <class 'function'>, but the keyword function is not defined.