BUG: Avoid sentinel-infinity comparison problems (#13445)
The problem causing #13445 ultimately traces to the fact that our Infinity/NegInfinity objects were greater than/less than themselves, which violates an assumption numpy makes when sorting.  This was separately reported as https://github.com/numpy/numpy/issues/7934, but we can fix and test downstream as well.
- [X] closes #13445
- [X] tests added / passed
- [X] passes `git diff upstream/master | flake8 --diff`
- [X] whatsnew entry
