remove period_helper from non-period reqs
There are a bunch of other reqs that _could_ be removed (e.g. every direct reference to src/util, all of `lib_depends`), but at least `period_helper` it's obvious _why_ it belongs where it does.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
