(py) Look into marking the module as GIL-less

[vladfrangu]

https://pyo3.rs/v0.25.1/free-threading.html

It looks like impit doesn't need the GIL at all [unless I'm missing something], so we can potentially look into also marking the module as supporting GIL-less runtimes (?).

[Mantisus]

We have GIL-blocking calls using Python::with_gil in CookieStore and ImpitPyResponse.

Therefore, this will require some further investigation using Python 3.14 after its release. In Python 3.14, free-threading will come out of experimental status.