TypeError: sum() got an unexpected keyword argument 'skipna'
#### Code Sample, a copy-pastable example if possible

```python
df = pd.DataFrame(index=np.arange(10), columns=np.arange(5), dtype=float)

df = 
    0   1   2   3   4
0 NaN NaN NaN NaN NaN
1 NaN NaN NaN NaN NaN
2 NaN NaN NaN NaN NaN
3 NaN NaN NaN NaN NaN
4 NaN NaN NaN NaN NaN
5 NaN NaN NaN NaN NaN
6 NaN NaN NaN NaN NaN
7 NaN NaN NaN NaN NaN
8 NaN NaN NaN NaN NaN
9 NaN NaN NaN NaN NaN

df.groupby(pd.Series(['a', 'a', 'b', 'b', 'b']), axis=1).agg('sum', skipna=True)
```
#### Problem description
The above call to `agg` gives 

```python
KeyError: 'a'
```

This is, because here:
https://github.com/pandas-dev/pandas/blob/67ee16a283b826b4adb07e5ca64a5206d242acaf/pandas/core/groupby/groupby.py#L1376
we are trying to access a new column name (`'a'`) in the original DataFrame.

It only occurs, when no `_cython_agg_general` is possible, e.g., when keyword argument `skipna` is given to `agg`. Without `skipna` argument the expected output below will be produced.
#### Expected Output
```
df = 
     a    b
0  0.0  0.0
1  0.0  0.0
2  0.0  0.0
3  0.0  0.0
4  0.0  0.0
5  0.0  0.0
6  0.0  0.0
7  0.0  0.0
8  0.0  0.0
9  0.0  0.0
```

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

<details>

```
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.8.final.0
python-bits      : 64
OS               : Windows
OS-release       : 7
machine          : AMD64
processor        : Intel64 Family 6 Model 42 Stepping 7, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : en
LOCALE           : None.None

pandas           : 0.25.1
numpy            : 1.16.4
pytz             : 2018.9
dateutil         : 2.8.0
pip              : 19.2.3
setuptools       : 40.8.0
Cython           : 0.29.7
pytest           : 4.3.1
hypothesis       : None
sphinx           : 2.1.2
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10
IPython          : 7.8.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.1
numexpr          : 2.6.9
odfpy            : None
openpyxl         : 2.6.1
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.1
sqlalchemy       : 1.3.1
tables           : 3.5.2
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
```

</details>
