ENH:AttributeError: 'SeriesGroupBy' object has no attribute 'kurtosis'
#### Code Sample, a copy-pastable example

```python
df = pd.DataFrame({'birb':['Falcon', 'Falcon', 'Falcon', 'Falcon',
                           'Parrot', 'Parrot', 'Parrot', 'Parrot'],
                   'value': [390., 350.,390., 350.,
                             30., 20., 30., 20.]})
df.groupby('birb').aggregate({'value': ['count', 'mean', 'skew']})

```

#### Problem description

Code gives error: `AttributeError: 'SeriesGroupBy' object has no attribute 'kurtosis'` instead of correct group-wise kurtosis.
Skew works ok.

#### Expected Output

```
       value            
       count   mean kurtosis
birb                    
Falcon     4  370.0  -6.0
Parrot     4   25.0  -6.0
```
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : 7d32926db8f7541c356066dcadabf854487738de
python           : 3.7.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.0-66-generic
Version          : #74~18.04.2-Ubuntu SMP Fri Feb 5 11:17:31 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_IE.UTF-8
LOCALE           : en_IE.UTF-8

pandas           : 1.2.2
numpy            : 1.17.5
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.0.1
setuptools       : 49.2.1
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           : 2.11.2
IPython          : 7.20.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.4
numexpr          : None
odfpy            : None
openpyxl         : 3.0.6
pandas_gbq       : None
pyarrow          : 3.0.0
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : 0.52.0

</details>
