BLD: 3.6.1 ABI compat
 - [x] closes #15961
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [ ] whatsnew entry

So I _think_ this works - locally was able to build a wheel with `3.6.1` and import it into a `3.6.0` environment, which was failing before this patch.