Fix attributes_to_get type stub in models.pyi (#746)

Author: sunaoka

pynamodb/models.pyi

@@ -42,7 +42,7 @@ class Model(metaclass=MetaModel):
     def save(self, condition: Optional[Condition] = ...) -> Dict[str, Any]: ...
     def refresh(self, consistent_read: bool = ...): ...
     @classmethod
-    def get(cls: Type[_T], hash_key: KeyType, range_key: Optional[KeyType] = ..., consistent_read: bool = ...) -> _T: ...
+    def get(cls: Type[_T], hash_key: KeyType, range_key: Optional[KeyType] = ..., consistent_read: bool = ..., attributes_to_get: Optional[Sequence[Text]] = ...) -> _T: ...
     @classmethod
     def from_raw_data(cls: Type[_T], data) -> _T: ...