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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18825?src=pr&el=h1) Report
> Merging [#18825](https://codecov.io/gh/pandas-dev/pandas/pull/18825?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cdebcf374598e9141df7dc23bc75c9528e92c004?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18825/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18825?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18825      +/-   ##
==========================================
+ Coverage   91.59%   91.59%   +<.01%     
==========================================
  Files         150      150              
  Lines       48959    48959              
==========================================
+ Hits        44843    44845       +2     
+ Misses       4116     4114       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.96% <ø> (ø)` | :arrow_up: |
| #single | `41.13% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18825?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18825/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.9% <0%> (+0.21%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18825?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18825?src=pr&el=footer). Last update [cdebcf3...ddb5247](https://codecov.io/gh/pandas-dev/pandas/pull/18825?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hello @hexgnu! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on December 23, 2017 at 20:56 Hours UTC I pushed a fix. ping on green. Awesome thanks @jreback looks like it all passed. thanks @hexgnu 
keep em coming!