fix Self import in MPTAmount

xrpl/models/amounts/mpt_amount.py

@@ -3,7 +3,9 @@
 from __future__ import annotations
 
 from dataclasses import dataclass
-from typing import Dict, Self, Union
+from typing import Dict, Union
+
+from typing_extensions import Self
 
 from xrpl.models.base_model import BaseModel
 from xrpl.models.required import REQUIRED