BUG: Fix nansum overflow on Windows with bottleneck
Bottleneck used in nansum produces overflow error on Windows. Temporary fix that could be undone once the error in bottleneck is fixed and released, to avoid platform specificity.

 - [x] closes #15453 
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry