BUG: .isin(...) now coerces sets to lists
- [ ] closes #12988
- [ ] tests added, passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] `comp.isin(values)` now coerces `values` from a set into a list. This was failing when `comp` is a datetime dtype and `values` is a set, where `.isin` will try to run `to_datetime(values)` on the unordered set and fail.

tests!
 I swear `git add` is playing games with me... sorry about that.
 How to kill an underway Travis job?
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13014?src=pr) is **83.91%**

> Merging [#13014](https://codecov.io/gh/pydata/pandas/pull/13014?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **+<.01%**

``` diff
@@             master     #13014   diff @@
==========================================
  Files           136        136          
  Lines         49908      49920    +12   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          41876      41888    +12   
  Misses         8032       8032          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by 0294c48
 @ch41rmn gr8. can you add a whatsnew note in bug fixes. ping when pushed.
 looks good. can you add a whatsnew note. ping when pushed.
 thanks @ch41rmn I merged this. hoping to get the release out shortly.
 Thanks for merging @jreback. Apologies I didn't get to adding the whatsnew note in time, have been away for the weekend.
