BUG: interpolate: fix missing free-threading support in _dierckxmodule.cc

Also clean up an unnecessary usage of `py3_dep` in the build declaration
for `_dierckx`.

Author: rgommers

scipy/interpolate/meson.build

@@ -144,7 +144,7 @@ py3.extension_module('_bspl',
 py3.extension_module('_dierckx',
     ['src/_dierckxmodule.cc'],
     include_directories: 'src/',
-    dependencies: [py3_dep, np_dep, __fitpack_dep],
+    dependencies: [np_dep, __fitpack_dep],
     link_args: version_link_args,
     install: true,
     subdir: 'scipy/interpolate'

scipy/interpolate/src/_dierckxmodule.cc

@@ -754,7 +754,18 @@ static struct PyModuleDef dierckxmodule = {
 PyMODINIT_FUNC
 PyInit__dierckx(void)
 {
+    PyObject *module;
+
     import_array();
 
-    return PyModule_Create(&dierckxmodule);
+    module = PyModule_Create(&dierckxmodule);
+    if (module == NULL) {
+        return NULL;
+    }
+
+#if Py_GIL_DISABLED
+    PyUnstable_Module_SetGIL(module, Py_MOD_GIL_NOT_USED);
+#endif
+
+    return module;
 }