c-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 testing this config.

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.