calling `mean` on a `DataFrameGroupBy` with `Int64` dtype results in `TypeError` 
```python
import pandas as pd

df = pd.DataFrame({
    'a' : [0,0,1,1,2,2,3,3],
    'b' : [1,2,3,4,5,6,7,8]
},
dtype='Int64')

df.groupby('a').mean()

```
#### Problem description

Using the new nullable integer data type, calling `mean` after grouping results in a `TypeError`. Using `int64` dtype it works:

```python
import pandas as pd

df = pd.DataFrame({
    'a' : [0,0,1,1,2,2,3,3],
    'b' : [1,2,3,4,5,6,7,8]
},
dtype='int64')

print(df.groupby('a').mean())
```

as does keeping `Int64` dtype but taking a single column to give a `SeriesGroupBy`:

```python
import pandas as pd

df = pd.DataFrame({
    'a' : [0,0,1,1,2,2,3,3],
    'b' : [1,2,3,4,5,6,7,8]
},
dtype='Int64')

print(df.groupby('a')['b'].mean())
```

The error does not occur when calling `min`, `max` or `first`, but does also occur with `median` and `std`.

#### Expected Output

```
     b
a     
0  1.5
1  3.5
2  5.5
3  7.5
```


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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.3.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-74-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.1
dateutil         : 2.8.0
pip              : 19.1.1
setuptools       : 41.0.1
Cython           : None
pytest           : 5.3.4
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.3.3
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.5.0
pandas_datareader: None
bs4              : 4.8.1
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.3.3
matplotlib       : 3.1.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.4
pyxlsb           : None
s3fs             : None
scipy            : 1.3.0
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
numba            : None
</details>
