reorganise code

Signed-off-by: Zhiyuan Chen <[email protected]>

chanfig/flat_dict.py

@@ -302,6 +302,9 @@ def __delattr__(self, name: Any) -> None:
         except KeyError:
             raise AttributeError(f"'{self.__class__.__name__}' object has no attribute '{name}'") from None
 
+    def __missing__(self, name: Any) -> Any:  # pylint: disable=R1710
+        raise KeyError(name)
+
     def getattr(self, name: str, default: Any = Null) -> Any:
         r"""
         Get attribute of `FlatDict`.
@@ -428,9 +431,6 @@ def hasattr(self, name: str) -> bool:
         except AttributeError:
             return False
 
-    def __missing__(self, name: Any) -> Any:  # pylint: disable=R1710
-        raise KeyError(name)
-
     def dict(self, cls: Callable = dict) -> Mapping:
         r"""
         Convert `FlatDict` to other `Mapping`.