39 fix signature post init

[eliegoudout]

Fixes mismatch between __signature__ property and actual behaviour.
Now runtime behaviour matches __signature__.

Example:

>>> class A(ParamClass):
...     x: int = 0
...     @classmethod
...     def __post_init__(cls, y: int, /) -> None:
...         print(cls.x + y)
...
>>> signature(A)
<Signature (post_init_args=[], /, *, x: int = 0)>
>>> A([0], x=1)
0
A(x=1)

---

Closes #39.