Add Python 3.14 free-threading build support

[ericbuehl]

With the release of Python 3.14, the new free-threaded (no-GIL) mode is becoming more widely used.

It would be great if line-profiler could provide prebuilt wheels and confirm compatibility with python3.14t (the free-threaded ABI).

[Erotemic]

This would be nice, but I'm not sure how difficult this would be. I'm not planning to implement this, but I'll work with anyone who wants to contribute the feature.