ENH: implement TimedeltaArray/TimedeltaIIndex sum, median, std
The signatures match those in PandasArray.

- [x] closes #25335
If that’s it then just could just add return None at the end of that func

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> On Aug 27, 2019, at 7:41 PM, jbrockmendel <notifications@github.com> wrote:
> 
> @jbrockmendel commented on this pull request.
> 
> In pandas/core/arrays/timedeltas.py:
> 
> > @@ -384,6 +386,49 @@ def astype(self, dtype, copy=True):
>              return self
>          return dtl.DatetimeLikeArrayMixin.astype(self, dtype, copy=copy)
>  
> +    def sum(
> $ mypy pandas/core/arrays/timedeltas.py
> pandas/core/indexes/base.py:257: error: Missing return statement
> 
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 > If that’s it then just could just add return None at the end of that func

Looks like that func is `Index.__new__` and the complaint is about the branch that calls `cls._scalar_data_error(data)`, which raises ahh I see you are closing an issue (xref my last comment), can you add a whatsnew note describing the change. Do any of these (median) need docstrings? I believe that `mean` will be inherited.

May need to add these to the `Attributes` section of the TimedeltaArray docstring, and add to the list in the API docs. this got lost but I think was ok, can you rebase and we'll look again. rebased thanks @jbrockmendel 

I guess this could have a whatsnew note, can you in future / other PR.