Fix seg. fault when accessing ConstantObject extra

[FrancescoCeruti]

This fixes (#9) a segmentation fault that started happening with python 3.12, changes in the PyLongObject implementation have broken some pointer arithmetic, which, from what I understand, is not necessary.

Probably someone with a better understanding of C should check this :)

I've tested with 3.12, 3.11 and 3.7.

[Houston4444]

Hi !
Is there still anybody maintaining this lib ?
If yes, it would be better to say you don't want to maintain it anymore.
The absence of a response generates programs that remain broken while awaiting a decision.
If you say you give up, someone forks and fix the project and everyone follows.