TST: Remove pow test in expressions
These are already skipped for NumPy>=1.12, and buggy for NumPy
1.10.4

cc @jreback 

This will be backported.

closes #18992 
great.  you need this with newest conda though. I added a commit to fix conda I don't think we need to backport the conda fix though. This should have passed on master (without testing 1.10.0), and when I add it to the backport PR it will be tested. right my point is if it DOES pass on master that will fix it. But it IS not tested on master currently. So I would merge this on master and just cherry-pick your 1 commit in the backport. (your 1 commit won't fail on master because of the pinned conda version) Sounds good. @TomAugspurger this looks good to merge & backport (the 1 commit)