BUG: Series.is_unique has extra output if contains objects with __ne__ defined


- [x] closes #20661
- [x] tests added / passed
  - series/test/series/test_analytics.py:test_is_unique_class_ne
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Simple change from `_checknan` to `checknull` in pandas/_libs/hashtable_class_helper.pxi.in

Checked performance, and there is no difference.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20691?src=pr&el=h1) Report
> Merging [#20691](https://codecov.io/gh/pandas-dev/pandas/pull/20691?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ad5affb6417c26bc7f3bc10668f0cf63fd867940?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20691/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/20691?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #20691   +/-   ##
=======================================
  Coverage   91.84%   91.84%           
=======================================
  Files         153      153           
  Lines       49275    49275           
=======================================
  Hits        45255    45255           
  Misses       4020     4020
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <ø> (ø)` | :arrow_up: |
| #single | `41.91% <ø> (ø)` | :arrow_up: |


------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20691?src=pr&el=footer). Last update [ad5affb...ee71a90](https://codecov.io/gh/pandas-dev/pandas/pull/20691?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback Removing checknan works, but failure was due to a network area with Amazon S3.  How can I get the travis run to be done again? thanks @Dr-Irv 

yeah sometimes the builds fail for extraneous reasons. you can push a dummy commit if you want, e.g.

``git commit --amend -C HEAD``
