BUG: Rounding on float16 type not working
- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

#### Problem description

I'm trying to round a column to 4 decimal places to store them in the DB. I must use `np.float16` type to work with as I need the smallest possible precision, but it's not working (see *output* section), if I don't cast the values and leave them as `np.float64` type It'll work correctly.
I think it's a bug of Pandas as if I run:

```python
np.array([0.37324, 0.12321, 0.23]).astype(np.float16).round(4)
```
It works as expected, so It doesn't seem to be a problem for Numpy.

#### Code Sample

```python
import pandas as pd
import numpy as np
df = pd.DataFrame(np.array([0.37324, 0.12321, 0.23]), columns=['A'])
df.astype(np.float16).round(4)
```

Running:

```python
pd.DataFrame(np.array([0.37324, 0.12321, 0.23]).astype(np.float16), columns=['A']).round({'A': 4})
```
Doesn't work either as it has the same output.

#### Current Output

```
          A
0  0.373291
1  0.123230
2  0.229980
```

#### Expected Output

```
          A
0  0.3732
1  0.1232
2  0.23
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-62-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : es_AR.UTF-8
LOCALE           : es_AR.UTF-8
pandas           : 1.0.5
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 18.1
setuptools       : 40.8.0
Cython           : None
pytest           : 5.3.5
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.5
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None


</details>
