Make Message.__getattribute__ invisible to type checkers (#359)

This lets linters know that we shouldn't access fields that aren't actually defined

src/betterproto/__init__.py

@@ -18,6 +18,7 @@ from datetime import (
     timezone,
 )
 from typing import (
+    TYPE_CHECKING,
     Any,
     Callable,
     Dict,
@@ -693,6 +694,8 @@ class Message(ABC):
         ]
         return f"{self.__class__.__name__}({', '.join(parts)})"
 
+    if not TYPE_CHECKING:
+
         def __getattribute__(self, name: str) -> Any:
             """
             Lazily initialize default values to avoid infinite recursion for recursive