`sysconfig.get_config_var('Py_GIL_DISABLED')` returns `None`

> It seems like 
> ```py
> sysconfig.get_config_var('Py_GIL_DISABLED')
> ```
> returns `None` (should return `1`) 

 _Originally posted by @ShaharNaveh in [#6428](https://github.com/RustPython/RustPython/pull/6428/changes#r2614589810)_