bpo-1635741: Port faulthandler module to multiphase initialization (GH-21294)

diff --git a/Modules/faulthandler.c b/Modules/faulthandler.c
index e7a2850..67fe1ca 100644
--- a/Modules/faulthandler.c
+++ b/Modules/faulthandler.c
@@ -1291,59 +1291,55 @@
     {NULL, NULL}  /* sentinel */
 };
 
+static int
+PyExec_faulthandler(PyObject *module) {
+    /* Add constants for unit tests */
+#ifdef MS_WINDOWS
+    /* RaiseException() codes (prefixed by an underscore) */
+    if (PyModule_AddIntConstant(module, "_EXCEPTION_ACCESS_VIOLATION",
+                                EXCEPTION_ACCESS_VIOLATION)) {
+        return -1;
+    }
+    if (PyModule_AddIntConstant(module, "_EXCEPTION_INT_DIVIDE_BY_ZERO",
+                                EXCEPTION_INT_DIVIDE_BY_ZERO)) {
+        return -1;
+    }
+    if (PyModule_AddIntConstant(module, "_EXCEPTION_STACK_OVERFLOW",
+                                EXCEPTION_STACK_OVERFLOW)) {
+        return -1;
+    }
+
+    /* RaiseException() flags (prefixed by an underscore) */
+    if (PyModule_AddIntConstant(module, "_EXCEPTION_NONCONTINUABLE",
+                                EXCEPTION_NONCONTINUABLE)) {
+        return -1;
+    }
+    if (PyModule_AddIntConstant(module, "_EXCEPTION_NONCONTINUABLE_EXCEPTION",
+                                EXCEPTION_NONCONTINUABLE_EXCEPTION)) {
+        return -1;
+    }
+#endif
+    return 0;
+}
+
+static PyModuleDef_Slot faulthandler_slots[] = {
+    {Py_mod_exec, PyExec_faulthandler},
+    {0, NULL}
+};
+
 static struct PyModuleDef module_def = {
     PyModuleDef_HEAD_INIT,
-    "faulthandler",
-    module_doc,
-    0, /* non-negative size to be able to unload the module */
-    module_methods,
-    NULL,
-    faulthandler_traverse,
-    NULL,
-    NULL
+    .m_name = "faulthandler",
+    .m_doc = module_doc,
+    .m_methods = module_methods,
+    .m_traverse = faulthandler_traverse,
+    .m_slots = faulthandler_slots
 };
 
 PyMODINIT_FUNC
 PyInit_faulthandler(void)
 {
-    PyObject *m = PyModule_Create(&module_def);
-    if (m == NULL)
-        return NULL;
-
-    /* Add constants for unit tests */
-#ifdef MS_WINDOWS
-    /* RaiseException() codes (prefixed by an underscore) */
-    if (PyModule_AddIntConstant(m, "_EXCEPTION_ACCESS_VIOLATION",
-                                EXCEPTION_ACCESS_VIOLATION)) {
-        goto error;
-    }
-    if (PyModule_AddIntConstant(m, "_EXCEPTION_INT_DIVIDE_BY_ZERO",
-                                EXCEPTION_INT_DIVIDE_BY_ZERO)) {
-        goto error;
-    }
-    if (PyModule_AddIntConstant(m, "_EXCEPTION_STACK_OVERFLOW",
-                                EXCEPTION_STACK_OVERFLOW)) {
-        goto error;
-    }
-
-    /* RaiseException() flags (prefixed by an underscore) */
-    if (PyModule_AddIntConstant(m, "_EXCEPTION_NONCONTINUABLE",
-                                EXCEPTION_NONCONTINUABLE)) {
-        goto error;
-    }
-    if (PyModule_AddIntConstant(m, "_EXCEPTION_NONCONTINUABLE_EXCEPTION",
-                                EXCEPTION_NONCONTINUABLE_EXCEPTION)) {
-        goto error;
-    }
-#endif
-
-    return m;
-
-#ifdef MS_WINDOWS
-error:
-    Py_DECREF(m);
-    return NULL;
-#endif
+    return PyModuleDef_Init(&module_def);
 }
 
 static int