BUG: repr of np.datetime64('NaT') in Series/DataFrame with dtype object
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Discovered while investigating #12499

Before:

```
In [1]: pd.Series([np.datetime64('NaT')], dtype=object)
Out[1]:
0    NaN
dtype: object
```
After:

```
In [1]: pd.Series([np.datetime64('NaT')], dtype=object)
Out[1]:
0    NaT
dtype: object
```
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25445?src=pr&el=h1) Report
> Merging [#25445](https://codecov.io/gh/pandas-dev/pandas/pull/25445?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/aa084162bcaa7ce0efdc044bc8077f6bfca70674?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25445/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25445?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25445      +/-   ##
==========================================
+ Coverage   91.74%   91.74%   +<.01%     
==========================================
  Files         173      173              
  Lines       52923    52924       +1     
==========================================
+ Hits        48554    48556       +2     
+ Misses       4369     4368       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.31% <100%> (ø)` | :arrow_up: |
| #single | `41.73% <50%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25445?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/25445/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `97.99% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25445/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.66% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25445?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/25445?src=pr&el=footer). Last update [aa08416...b7aaa02](https://codecov.io/gh/pandas-dev/pandas/pull/25445?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25445?src=pr&el=h1) Report
> Merging [#25445](https://codecov.io/gh/pandas-dev/pandas/pull/25445?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7721f7009249e2b1fa2647526114e7c423fef4ea?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25445/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25445?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25445      +/-   ##
==========================================
+ Coverage   91.77%   91.79%   +0.01%     
==========================================
  Files         175      174       -1     
  Lines       52606    52539      -67     
==========================================
- Hits        48280    48229      -51     
+ Misses       4326     4310      -16
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.35% <100%> (+0.02%)` | :arrow_up: |
| #single | `41.88% <75%> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25445?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/25445/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `97.88% <100%> (-0.11%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/25445/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `75% <0%> (-12.5%)` | :arrow_down: |
| [pandas/core/config\_init.py](https://codecov.io/gh/pandas-dev/pandas/pull/25445/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnX2luaXQucHk=) | `96.96% <0%> (-2.24%)` | :arrow_down: |
| [pandas/core/computation/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/25445/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vY29tbW9uLnB5) | `89.47% <0%> (-0.53%)` | :arrow_down: |
| [pandas/compat/pickle\_compat.py](https://codecov.io/gh/pandas-dev/pandas/pull/25445/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9waWNrbGVfY29tcGF0LnB5) | `69.13% <0%> (-0.38%)` | :arrow_down: |
| [pandas/compat/numpy/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/25445/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9udW1weS9fX2luaXRfXy5weQ==) | `92.85% <0%> (-0.25%)` | :arrow_down: |
| [pandas/plotting/\_style.py](https://codecov.io/gh/pandas-dev/pandas/pull/25445/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19zdHlsZS5weQ==) | `77.17% <0%> (-0.25%)` | :arrow_down: |
| [pandas/core/computation/engines.py](https://codecov.io/gh/pandas-dev/pandas/pull/25445/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vZW5naW5lcy5weQ==) | `88.52% <0%> (-0.19%)` | :arrow_down: |
| [pandas/plotting/\_timeseries.py](https://codecov.io/gh/pandas-dev/pandas/pull/25445/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL190aW1lc2VyaWVzLnB5) | `65.28% <0%> (-0.18%)` | :arrow_down: |
| [pandas/io/excel/\_util.py](https://codecov.io/gh/pandas-dev/pandas/pull/25445/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsL191dGlsLnB5) | `87.5% <0%> (-0.18%)` | :arrow_down: |
| ... and [40 more](https://codecov.io/gh/pandas-dev/pandas/pull/25445/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25445?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/25445?src=pr&el=footer). Last update [7721f70...8d8abd1](https://codecov.io/gh/pandas-dev/pandas/pull/25445?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 let's wait till we merge the min numpy 1.13 and then revist Address comments and all green thanks @mroeschke 