+    result = PyObject_CallFunction((PyObject*)&PyType_Type, "s(O){OOOO}",
+                    type, base,
+                    _PyUnicode_FromId(&PyId__fields), fnames,
+                    _PyUnicode_FromId(&PyId___module__),
+                    _PyUnicode_FromId(&PyId__ast));

You should check if _PyUnicode_FromId() returns NULL if it was the first call and the UTF-8 decode failed to allocate memory.

You might initialize all these identifiers (and check for errors) in init_types() to avoid having to check for errors each time they are used.