CI: update tests for numpy 1.20 change to floordiv
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Looks like https://github.com/numpy/numpy/pull/16161 changed numpy's floordiv behavior