groupby.quantile() fails on Timedelta columns
#### Code Sample, groupby quantile example

```python
# groupby quantile example
import pandas as pd

print(f"Pandas {pd.__version__}")

df = pd.DataFrame({
    'idx': [1, 1, 2],
    'start_date': ['2019-10-02', '2019-10-04', '2019-10-05'],
    'arrival_date': ['2019-11-02', '2019-10-14', '2019-10-15']})

for c in ['start_date', 'arrival_date']:
    df[c] = pd.to_datetime(df[c])

df['delta'] = df['arrival_date'] - df['start_date']

print(df.groupby('idx')[['delta']].quantile(.9))
```
#### Problem description

The code example used to work in 0.21.1.
In 0.25 I obtain a "TypeError: No matching signature found"

#### Expected Output
Pandas 0.21.1

               delta
idx
1   28 days 21:36:00
2   10 days 00:00:00


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

<details>


INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.8.final.0
python-bits      : 64
OS               : Linux
OS-release       : 3.10.0-693.el7.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            : 1.17.3
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 18.1
setuptools       : 40.8.0.post20191016
Cython           : 0.29.14
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
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
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None

</details>
