BUG: Fixed accessor for Categorical[Datetime]
Closes https://github.com/pandas-dev/pandas/issues/19468

No release note, since this was only a regression on master.

cc @jschendel.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19469?src=pr&el=h1) Report
> Merging [#19469](https://codecov.io/gh/pandas-dev/pandas/pull/19469?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/238499ab0a48a0ad4a2011e2ce1c6a02c86124eb?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19469/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19469?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19469      +/-   ##
==========================================
- Coverage   91.62%   91.59%   -0.03%     
==========================================
  Files         150      150              
  Lines       48724    48728       +4     
==========================================
- Hits        44642    44633       -9     
- Misses       4082     4095      +13
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.96% <100%> (-0.03%)` | :arrow_down: |
| #single | `41.74% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19469?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/accessors.py](https://codecov.io/gh/pandas-dev/pandas/pull/19469/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9hY2Nlc3NvcnMucHk=) | `90.19% <100%> (+0.19%)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19469/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/19469/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.42% <0%> (-0.16%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/19469/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.91% <0%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19469/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `83.85% <0%> (+0.2%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19469?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/19469?src=pr&el=footer). Last update [238499a...ac95946](https://codecov.io/gh/pandas-dev/pandas/pull/19469?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks, LGTM lgtm. merge on green.