no `None` default for `saqc._wrap.inner` arguments `to_mask` and `flag`

we could provide a custom None like so

class UnsetType:
    instance = None
    def __new__(cls):
        if cls.instance is None:
            cls.instance = super().__new__(cls)
        return cls.instance
    def __bool__(self):
        return False
    
Unset = UnsetType()

and use it as so:

def foo(arg: UnsetType | None | int = Unset)
    if arg is Unset:
        arg = 10  # arg was not given
    if arg is None:
        arg = 12  # arg was set to None by user
    ...