netCDF4 / cftime real_datetime object is not supported
#### Code Sample, a copy-pastable example if possible

```python
import pandas, numpy, netCDF4
time=netCDF4.num2date([0,1,2,3,4,5,500,1000], 'days since 1801-10-01')
pandas.Series(numpy.arange(len(time)), index=time)
```
#### Problem description

NetCDF4 is giving us arrays with cftime.real_datetime objects in them, which leads to the following error:

```
>>> import pandas, numpy, netCDF4
>>> time=netCDF4.num2date([0,1,2,3,4,5,500,1000], 'days since 1801-10-01')
>>> pandas.Series(numpy.arange(len(time)), index=time)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/users/timo/oggm_venv/lib/python3.7/site-packages/pandas/core/series.py", line 183, in __init__
    index = _ensure_index(index)
  File "/home/users/timo/oggm_venv/lib/python3.7/site-packages/pandas/core/indexes/base.py", line 4974, in _ensure_index
    return Index(index_like)
  File "/home/users/timo/oggm_venv/lib/python3.7/site-packages/pandas/core/indexes/base.py", line 417, in __new__
    name=name, **kwargs)
  File "/home/users/timo/oggm_venv/lib/python3.7/site-packages/pandas/core/indexes/datetimes.py", line 399, in __new__
    yearfirst=yearfirst)
  File "/home/users/timo/oggm_venv/lib/python3.7/site-packages/pandas/core/tools/datetimes.py", line 467, in to_datetime
    result = _convert_listlike(arg, box, format)
  File "/home/users/timo/oggm_venv/lib/python3.7/site-packages/pandas/core/tools/datetimes.py", line 368, in _convert_listlike
    require_iso8601=require_iso8601
  File "pandas/_libs/tslib.pyx", line 492, in pandas._libs.tslib.array_to_datetime
  File "pandas/_libs/tslib.pyx", line 544, in pandas._libs.tslib.array_to_datetime
AttributeError: 'real_datetime' object has no attribute 'nanosecond'
```

#### Expected Output

Well, not an error would be nice.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.1.final.0
python-bits: 64
OS: Linux
OS-release: 4.14.78-gentoo
machine: x86_64
processor: Intel(R) Xeon(R) CPU E5-2623 v4 @ 2.60GHz
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.4
pytest: 3.9.3
pip: 18.1
setuptools: 40.5.0
Cython: 0.29
numpy: 1.15.3
scipy: 1.1.0
pyarrow: None
xarray: 0.10.9
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.5
pytz: 2018.7
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 3.0.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>

netCDF4 version is 1.4.2
