BUG: Series/Index contains NaT with object dtype comparison incorrect
- [x] closes #9005 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

`Series` and `Index` compares `NaT == NaT` as `True` if it has `object` dtype. 

```
pd.Series([pd.NaT])
#0   NaT
# dtype: datetime64[ns]

# OK
pd.Series([pd.NaT]) == pd.NaT
#0    False
# dtype: bool

# NG in object dtype
pd.Series([pd.NaT], dtype=object) == pd.NaT
#0    True
# dtype: bool

# OK
pd.Index([pd.NaT]) == pd.NaT
array([False], dtype=bool)

# NG in object dtype
pd.Index([pd.NaT], dtype=object) == pd.NaT
# array([ True], dtype=bool)
```

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13592?src=pr) is **84.33%**

> Merging [#13592](https://codecov.io/gh/pydata/pandas/pull/13592?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **0.01%**

``` diff
@@             master     #13592   diff @@
==========================================
  Files           138        138          
  Lines         51165      51100    -65   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43142      43097    -45   
+ Misses         8023       8003    -20   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [1edc1df...f95dae7](https://codecov.io/gh/pydata/pandas/compare/1edc1df161f3274218fcd19c23663ea63386f105...f95dae7ad6648a68ee0f63450854f676e14badf4?src=pr)
 lgtm. Do we have an associated issue? I do recall this somewhere.
 Maybe #9005, though it looks to be fixed on current master (but added tests for native/object dtypes).
 Ah, found #9005 is not fixed yet. If `NaT` is on rhs (on current master):

```
pd.Series([pd.Timestamp('2011-01-01')]) > pd.Series([pd.NaT])
# 0    True
# dtype: bool
```

Already included the fix to this PR.
 thanks!
