ENH: use constant f32 eps, not np.finfo() during import
NumPy docs for `np.finfo()` say not to call it during import (at module scope).
It's a relatively expensive call, and it modifies the GIL state.
Now we just hard-code it, because it is always the value anyway.
This avoids touching the GIL at import, which helps avoid deadlocks in practice.

 - [x] closes #14641
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry
