Fix groupby().count() for datetime columns
- [x] closes #13393
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

In which whatsnew file should the whatsnew entry go? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18167?src=pr&el=h1) Report
> Merging [#18167](https://codecov.io/gh/pandas-dev/pandas/pull/18167?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/93c755e0ea64b6039c8ecd55fc637d1dd67dbd6e?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/18167/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18167?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18167      +/-   ##
==========================================
+ Coverage    91.4%   91.41%   +<.01%     
==========================================
  Files         163      163              
  Lines       50073    50074       +1     
==========================================
+ Hits        45769    45773       +4     
+ Misses       4304     4301       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.21% <100%> (+0.02%)` | :arrow_up: |
| #single | `40.32% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18167?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/18167?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.04% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18167?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18167?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18167?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.2% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18167?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/18167?src=pr&el=footer). Last update [93c755e...073f22d](https://codecov.io/gh/pandas-dev/pandas/pull/18167?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/18167?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@5350330`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18167/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18167?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #18167   +/-   ##
=========================================
  Coverage          ?   91.38%           
=========================================
  Files             ?      163           
  Lines             ?    50068           
  Branches          ?        0           
=========================================
  Hits              ?    45754           
  Misses            ?     4314           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.19% <100%> (?)` | |
| #single | `40.33% <0%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18167?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/18167?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.04% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18167?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/18167?src=pr&el=footer). Last update [5350330...9eb429e](https://codecov.io/gh/pandas-dev/pandas/pull/18167?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I don't think the Travis Issue is related. thanks @watercrossing, nice PR!