BUG: casting column in place doesn't change data type
- [x] I have checked that this issue has not already been reported.

- [ ] 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.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
>>> a = pd.DataFrame({'a': [b'blah']})
>>> a.dtypes
a    object
dtype: object
>>>  a.a = a.a.astype(np.dtype("S8"))
>>> a.dtypes
a    object
dtype: object
>>> a['b'] = a.a.astype(np.dtype("S8"))
>>> a.dtypes
a    object
b       |S8
dtype: object
```

#### Problem description

Naively it seems like this should behave similarly to other data types. Trying the same example and casting int64 -> int32 works.

#### Expected Output

```python
>>> a = pd.DataFrame({'a': [b'blah']})
>>> a.dtypes
a    object
dtype: object
>>>  a.a = a.a.astype(np.dtype("S8"))
>>> a.dtypes
a       |S8
dtype: object
```

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

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.6.6-300.fc32.x86_64
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.3
numpy            : 1.18.4
pytz             : 2020.4
dateutil         : 2.8.0
pip              : 19.3.1
setuptools       : 41.6.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.1
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.12.0
pandas_datareader: 0.8.0
bs4              : None
bottleneck       : 1.2.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : 3.2.2
numexpr          : 2.7.1
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : 3.5.2
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.1.2
xlsxwriter       : None
