ValueError when summing over column level with skipna=False (Python 3 only)
#### Code Sample, a copy-pastable example if possible

```python
import numpy as np
import pandas as pd

df = pd.DataFrame(np.ones([2, 8]))
df.iloc[0, 0] = np.nan
df.columns = pd.MultiIndex.from_product([list('ab'), list('cd'), list('ef')])
df.sum(axis=1, level=[0, 1], skipna=False)
```
#### Problem description

In Python 3.6 a ValueError is raised: `ValueError: No axis named 1 for object type <class 'pandas.core.series.Series'>`. In Python 2.7 the above works as expected.

#### Expected Output

The same as in Python 2.7, namely the dataframe:

```
     a         b     
     c    d    c    d
0  NaN  2.0  2.0  2.0
1  2.0  2.0  2.0  2.0
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 32
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
pandas: 0.22.0
pytest: None
pip: 9.0.1
setuptools: 28.8.0
Cython: None
numpy: 1.14.0
scipy: 1.0.0
pyarrow: None
xarray: 0.10.0
IPython: 6.2.1
sphinx: None
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2018.3
blosc: None
bottleneck: 1.2.1
tables: None
numexpr: None
feather: None
matplotlib: 2.1.2
openpyxl: 2.5.0
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: None
lxml: 4.1.1
bs4: 4.6.0
html5lib: None
sqlalchemy: 1.2.2
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None

</details>
