A workaround for private __init__ by using name mangling.

private_init.py

#!/usr/bin/env python3

from __future__ import annotations

class Foo:
    class __Internal:
        pass

    def __init__(self, _: Type[__Internal]) -> None:
        pass

    @staticmethod
    def empty() -> Foo:
        return Foo(Foo.__Internal)

Foo.empty() # OK
Foo(Foo.__Internal) # NG: AttributeError: type object 'Foo' has no attribute '__Internal'