BUG make hashtable.unique support readonly arrays
This problem was brought up in
https://github.com/pandas-dev/pandas/issues/18773 and effectively comes
down to how Cython deals with readonly arrays. While it would be ideal
for Cython to fix the underlying problem in the meantime we can rely on
this.

- [x] closes #18773
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
