std and var units

[andrewgsavage]

https://github.com/lucascolley/pint-array/blob/main/src/pint_array/__init__.py#L674

the units on these should be delta units, they're wrong in pint
see https://github.com/hgrecco/pint/pull/1992/files

[lucascolley]

cool, thanks! By the way, I know that lots of the unit behaviour isn't correct yet, particularly the elementwise ops. On the bright side, we're almost passing all of array-api-tests now.