Remove from numpy cimport *
Replace these imports with explicit imports.

There are a couple of other similar things coming up, splitting them apart for easier review.

There are a couple of places with `cdef extern from "../src/headers/math.h":` but I don't think the path here makes sense.  The fact that it works anyway bothers me.  This PR changes one such occurrence as a demonstration, leaves the others for the next PR.

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