Free-threading: run with pytest-run-paralell

[crusaderky]

Mark the project as compatible with free-threading.

Stress-test the whole test suite (and, crucially, _speedups.so) by running it in parallel over multiple threads.
This aims to detect race conditions where functions may share a thread-unsafe global state.
Namely, in the future one may add a global cache, which carries thread-safety concerns.

[davidism]

Why is this "beta"? Why not "stable" or "resilient"?

[crusaderky]

Hi, please refer to this: https://py-free-threading.github.io/porting/#define-and-document-thread-safety-guarantees

1. Alpha - For experimentation and feedback only.
2. Beta - Free threaded usage is supported, but documentation of constraints and limitations may be incomplete.
3. Stable - Supported for production use, multithreaded use is tested, and thread safety issues are clearly documented.
4. Resilient - Fully supported and fully thread safe.

In general, I would not go beyond Beta status before having some feedback that everything works well in production environments.

[davidism]

I'm going to remove the classifier for now. I don't understand free threading enough to evaluate the "Beta", "Stable", and "Resilient" classifiers. Our intention is to support free threading, if we don't then that's a bug to report and help fix. We're such a simple extension that I don't think we needed to do anything to become thread safe.

The fact that we supply wheels for free threaded builds signals our support, as well as the fact that we build successfully as reported by https://ft-checker.com/?search=markupsafe. And these parallel tests pass as well.