Indicate support of running with GIL disabled, as per python-pillow#8216

Author: radarhere

src/_avif.c

@@ -940,5 +940,9 @@ PyInit__avif(void) {
         return NULL;
     }
 
+#ifdef Py_GIL_DISABLED
+    PyUnstable_Module_SetGIL(m, Py_MOD_GIL_NOT_USED);
+#endif
+
     return m;
 }