bpo-38138: Fix memory leak introduced by interned strings (GH-16053)

Interned string needs to be decref'd



https://bugs.python.org/issue38138



Automerge-Triggered-By: @matrixise

Python/ast.c

@@ -630,6 +630,7 @@ new_identifier(const char *n, struct compiling *c)
         PyObject *args[2] = {form, id};
         id2 = _PyObject_FastCall(c->c_normalize, args, 2);
         Py_DECREF(id);
+        Py_DECREF(form);
         if (!id2)
             return NULL;
         if (!PyUnicode_Check(id2)) {