BUG: error writing sparse DataFrame to HDF
- [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.

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

```python
import pandas as pd

filepath = r"<path where the file should be stored>"

data = [1,2,None,None,None,3]
sparse_data = pd.arrays.SparseArray(data)
df = pd.DataFrame(dict(a=sparse_data))

df.to_hdf(filepath, "data", format="t", data_columns=True)
```
```python
>>>  File "...\lib\site-packages\pandas\io\pytables.py", line 2323, in _get_atom
>>>    itemsize = dtype.itemsize  # type: ignore[attr-defined]
>>>  AttributeError: 'SparseDtype' object has no attribute 'itemsize'
```

#### Problem description

the code-example tells it all... saving a sparse-DataFrame to HDF raises an error


#### Expected Output

a properly exported DataFrame

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

<details>

INSTALLED VERSIONS
------------------
commit           : 2cb96529396d93b46abab7bbc73a208e708c642e
python           : 3.8.8.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.17763
machine          : AMD64
processor        : AMD64 Family 25 Model 33 Stepping 0, AuthenticAMD
byteorder        : little
LC_ALL           : None
LANG             : en
LOCALE           : German_Austria.1252

pandas           : 1.2.4
numpy            : 1.20.2
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.1.1
setuptools       : 49.6.0.post20210108
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : 4.0.2
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 3.0.1
IPython          : 7.24.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : 2021.06.0
fastparquet      : None
gcsfs            : None
matplotlib       : 3.4.1
numexpr          : 2.7.3
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.6.3
sqlalchemy       : None
tables           : 3.6.1
tabulate         : None
xarray           : 0.17.0
xlrd             : None
xlwt             : None
numba            : None

</details>
