Declare extension module to be free-threading compatible

[rgommers]

This is a follow-up to gh-94. The full test suite passes under thread-parallel testing with pytest-run-parallel and ThreadSanitizer. xxHash is thread-safe (Cyan4973/xxHash#133), and in src/_xxhash.c the update calls release the GIL already (Py_BEGIN_ALLOW_THREADS), so all looks good.

[ifduyue]

@rgommers thanks for contributing, could you solve the conflict?

[rgommers]

Of course, done!

[rgommers]

Thanks for the quick review @ifduyue. I considered also including the changes needed to build wheels for free-threaded Python (cp313t), but that looked slightly more involved. I'd like to do that in a follow-up though - mind if I open an issue covering all of free-threaded support and add wheel builds?

Another thing that could be useful to guard against regressions is to use pytest-run-parallel in a CI job. It may not be necessary though, the chance of a regression doesn't seem high given how stable is and the relatively small amount of C code.

[ifduyue]

@rgommers that's nice, go ahead if you want