FIX: const-correctness in numpy helpers
In python 3.7 the return type of PyUnicode_AsUTF8 changed
from (char *) to (const char *).

PyUnicode_FromString also takes (const char *) as input, also be
explicit about that.

https://bugs.python.org/issue28769
commit 2a404b63d48d73bbaa007d89efb7a01048475acd in cpython

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