Support free-threaded Python builds

[beer-psi]

Free-threading is no longer experimental as of Python 3.14, so I was wondering if it's possible for the library to support free-threaded builds.

[rushter]

We already have them.
https://pypi.org/project/selectolax/0.4.7/#files

[beer-psi]

They're free-threaded wheels, but they still enable GIL when imported. You need to also mark the module as freethreading-compatible.

$ uv run --with=selectolax -p 3.14t python
Installed 1 package in 7ms
Python 3.14.2 free-threading build (main, Dec  9 2025, 19:03:17) [Clang 21.1.4 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import selectolax
<frozen importlib._bootstrap>:491: RuntimeWarning: The global interpreter lock (GIL) has been enabled to load
module 'selectolax.lexbor', which has not declared that it can run safely without the GIL. To override this
behavior and keep the GIL disabled (at your own risk), run with PYTHON_GIL=0 or -Xgil=0.

[rushter]

I've enabled it for the dev build, but I'm not sure if it will work correctly.