Clarify types to allow None in indexing

I believe that the indexing specification allow `None` in a selection tuple, but the current typing does not specify this. I have changed the signature for `__getitem__` but did not change `__setitem__`. I am not sure where it is documented if the indexing specification applies to both equally.

Author: saulshanabrook

src/array_api_stubs/_draft/array_object.py

@@ -477,7 +477,7 @@ def __ge__(self: array, other: Union[int, float, array], /) -> array:
     def __getitem__(
         self: array,
         key: Union[
-            int, slice, ellipsis, Tuple[Union[int, slice, ellipsis], ...], array
+            int, slice, ellipsis, Tuple[Union[int, slice, ellipsis, None], ...], array
         ],
         /,
     ) -> array:
@@ -488,7 +488,7 @@ def __getitem__(
         ----------
         self: array
             array instance.
-        key: Union[int, slice, ellipsis, Tuple[Union[int, slice, ellipsis], ...], array]
+        key: Union[int, slice, ellipsis, Tuple[Union[int, slice, ellipsis, None], ...], array]
             index key.
 
         Returns