MAINT: add missing dunder methods to nditer type hints

The presence of __getattr__ is not sufficient to enable all protocols so explicitely add the dunder methods supported by nditer

Author: MatthieuDartiailh

numpy/__init__.pyi

@@ -172,6 +172,7 @@ from typing import (
     Generic,
     IO,
     Iterable,
+    Iterator,
     List,
     Mapping,
     NoReturn,
@@ -778,6 +779,21 @@ class nditer:
         buffersize: Any = ...,
     ) -> Any: ...
     def __getattr__(self, key: str) -> Any: ...
+    def __enter__(self) -> nditer: ...
+    def __exit__(
+        self,
+        exc_type: None | Type[BaseException],
+        exc_value: None | BaseException,
+        traceback: None | TracebackType,
+    ) -> None: ...
+    def __iter__(self) -> Iterator[Any]: ...
+    def __next__(self) -> Any: ...
+    def __len__(self) -> int: ...
+    def __copy__(self) -> nditer: ...
+    def __getitem__(self, index: SupportsIndex | slice) -> Any: ...
+    def __setitem__(self, index: SupportsIndex | slice, value: Any) -> None: ...
+    def __delitem__(self, key: SupportsIndex | slice) -> None: ...
+
 
 class poly1d:
     def __init__(