ext: declare GIL as not used

Without this, the GIL is enabled when loading the extension module since we
don't declare nogil compatibility.

Let's allow loading without the GIL so we can start meaningfully testing
running without the GIL.

We still don't formally support nogil interpreters.

Author: indygreg

c-ext/backend_c.c

@@ -134,6 +134,10 @@ void decompressoriterator_module_init(PyObject *mod);
 void frameparams_module_init(PyObject *mod);
 
 void zstd_module_init(PyObject *m) {
+#ifdef Py_GIL_DISABLED
+    PyUnstable_Module_SetGIL(m, Py_MOD_GIL_NOT_USED);
+#endif
+
     /* python-zstandard relies on unstable zstd C API features. This means
        that changes in zstd may break expectations in python-zstandard.
 

docs/news.rst

@@ -18,6 +18,7 @@ Version History
   <3.14, as cffi didn't implement nogil support until the 2.0 release. (#274)
 * Added CI coverage for free-threaded CPython 3.13 and 3.14. We do not yet
   formally support free-threaded builds. (#276)
+* The C and Rust backends now declare the GIL as unused.
 
 0.24.0 (released 2025-08-17)
 ============================

rust-ext/src/lib.rs

@@ -34,7 +34,7 @@ use exceptions::ZstdError;
 // and debian/changelog as well.
 const VERSION: &'static str = "0.25.0.dev0";
 
-#[pymodule]
+#[pymodule(gil_used = false)]
 fn backend_rust(py: Python, module: &Bound<'_, PyModule>) -> PyResult<()> {
     let features = PySet::new(
         py,