bpo-1635741: Port _string module to multi-phase init (GH-22148)

Port the _string extension module to the multi-phase initialization
API (PEP 489).

Author: vstinner

Misc/NEWS.d/next/Library/2020-09-08-13-51-16.bpo-1635741.wkPeoT.rst

@@ -0,0 +1,2 @@
+Port the ``_string`` extension module to the multi-phase initialization API
+(:pep:`489`).

Objects/unicodeobject.c

@@ -16243,20 +16243,16 @@ static PyMethodDef _string_methods[] = {
 
 static struct PyModuleDef _string_module = {
     PyModuleDef_HEAD_INIT,
-    "_string",
-    PyDoc_STR("string helper module"),
-    0,
-    _string_methods,
-    NULL,
-    NULL,
-    NULL,
-    NULL
+    .m_name = "_string",
+    .m_doc = PyDoc_STR("string helper module"),
+    .m_size = 0,
+    .m_methods = _string_methods,
 };
 
 PyMODINIT_FUNC
 PyInit__string(void)
 {
-    return PyModule_Create(&_string_module);
+    return PyModuleDef_Init(&_string_module);
 }