BUG: is_categorical_dtype returns False for Sparse[category, nan]
- [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
from pandas.api.types import is_categorical_dtype

s = pd.Series(
    ["a", "b", "c", "a", "b", "c"],
    dtype=pd.SparseDtype(pd.CategoricalDtype(['a', 'b', 'c']))
)

print(s.dtype)
# out: Sparse[category, nan]
print(is_categorical_dtype(s))
# out: False
```

#### Problem description
The `is_categorical_dtype` function returns `False` when sparse. This is inconsistent with other types (example for unsigned integer below).

```python
from pandas.api.types import is_unsigned_integer_dtype

s = pd.Series(
    [0, 1, 2, 3],
    name="pd_unsigned_integer",
    dtype=pd.SparseDtype(pd.UInt8Dtype(), pd.NA)
)
print(s.dtype)
# out: Sparse[UInt8, <NA>]
print(is_unsigned_integer_dtype(s))
# out: True
```

```python
s = pd.Series(
    ["a", "b", "c", "a", "b", "c"],
    dtype=pd.CategoricalDtype(['a', 'b', 'c'])
)

print(s.dtype)
# out: category
print(is_categorical_dtype(s))
# out: True
```

#### Expected Output

```python
print(is_categorical_dtype(s))
# out: True
```

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

<details>


INSTALLED VERSIONS
------------------
commit           : d9fff2792bf16178d4e450fe7384244e50635733
python           : 3.7.6.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.18362
machine          : AMD64
processor        : Intel64 Family 6 Model 158 Stepping 10, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.1.0
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 45.0.0.post20200113
Cython           : None
pytest           : 5.3.3
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.1.2
numexpr          : 2.7.1
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : 0.47.0

</details>
