BUG: Datetime subclasses are no longer datetime64 types
#### Code Sample, a copy-pastable example if possible

```python
>>> import pandas as pd
>>> from datetime import datetime
>>> class MyDatetime(datetime):
...             pass
>>> df_subclass = pd.DataFrame({"datetime": [MyDatetime(2018, 1, 1, 1, 1)]})
>>> df_subclass.dtypes
datetime    object
dtype: object

>>> df_dt = pd.DataFrame({"datetime": [datetime(2018, 1, 1, 1, 1)]})
>>> df_dt.dtypes
datetime    datetime64[ns]
dtype: object
```
#### Problem description

Since version 0.23.0 pandas treats subclasses of datetime  no longer as `datetime64[ns]`

#### Expected Output

Same behavior as in pre 0.23.0 versions, i.e. subclasses of datetime are of type `datetime64[ns]`

#### Output of ``pd.show_versions()``

<details>


INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 16.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.0
pytest: 3.4.0
pip: 9.0.1
setuptools: 38.5.1
Cython: 0.27.3
numpy: 1.14.3
scipy: None
pyarrow: 0.8.1.dev163+gb33dfd9
xarray: None
IPython: 6.2.1
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
