Python2 test_datetime_name_accessors failure with some locales
#### Problem description

Python2 **TestDatetime64.test_datetime_name_accessors[en_IL]** fails with `ValueError: unknown locale: en_IL`.  The same test passes in Python3.

By default, **tm.get_locales()** should only return locales that can be set without throwing an exception.

Compare the output in Python2:
```$ python2
Python 2.7.15 (default, May  1 2018, 05:55:50) 
[GCC 7.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pandas.util.testing as tm; tm.get_locales()
['af_ZA.UTF-8', 'C', 'en_US.UTF-8', 'de_AT.UTF-8', 'de_BE.UTF-8', 'de_CH.UTF-8', 'de_DE.UTF-8', 'de_IT.utf8', 'de_LI.UTF-8', 'de_LU.UTF-8', 'en_AG.UTF-8', 'en_AG.UTF-8', 'en_AU.UTF-8', 'en_BW.UTF-8', 'en_CA.UTF-8', 'en_DK.UTF-8', 'en_GB.UTF-8', 'en_HK.UTF-8', 'en_IE.UTF-8', 'en_IL', 'en_IL.utf8', 'en_IN.UTF-8', 'en_NG.UTF-8', 'en_NG.UTF-8', 'en_NZ.UTF-8', 'en_PH.UTF-8', 'en_SG.UTF-8', 'en_US.UTF-8', 'en_ZA.UTF-8', 'en_ZM.UTF-8', 'en_ZM.UTF-8', 'fr_CH.UTF-8', 'fr_FR.ISO8859-1', 'fr_FR.ISO8859-1', 'fr_FR.ISO8859-1', 'fr_FR.UTF-8', 'C', '']
```
with that in Python3:
```$ python3
Python 3.6.6 (default, Jun 27 2018, 14:44:17) 
[GCC 8.1.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pandas.util.testing as tm; tm.get_locales()
['af_ZA.UTF-8', 'C', 'en_US.UTF-8', 'de_AT.UTF-8', 'de_BE.UTF-8', 'de_CH.UTF-8', 'de_DE.UTF-8', 'de_IT.UTF-8', 'de_LI.UTF-8', 'de_LU.UTF-8', 'en_AG.UTF-8', 'en_AG.UTF-8', 'en_AU.UTF-8', 'en_BW.UTF-8', 'en_CA.UTF-8', 'en_DK.UTF-8', 'en_GB.UTF-8', 'en_HK.UTF-8', 'en_IE.UTF-8', 'en_IL.UTF-8', 'en_IL.UTF-8', 'en_IN.UTF-8', 'en_NG.UTF-8', 'en_NG.UTF-8', 'en_NZ.UTF-8', 'en_PH.UTF-8', 'en_SG.UTF-8', 'en_US.UTF-8', 'en_ZA.UTF-8', 'en_ZM.UTF-8', 'en_ZM.UTF-8', 'fr_CH.UTF-8', 'fr_FR.ISO8859-1', 'fr_FR.ISO8859-1', 'fr_FR.ISO8859-1', 'fr_FR.UTF-8', 'C', '']
```
The Python2 output contains `'en_IL', 'en_IL.utf8'` while the Python3 output contains `'en_IL.UTF-8', 'en_IL.UTF-8'`.

#### Expected Output

The same locales should be returned in Python2 and Python3.

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.6.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-29-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.23.3
pytest: 3.3.2
pip: 9.0.1
setuptools: 39.2.0
Cython: 0.28.4
numpy: 1.14.5
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 5.5.0
sphinx: 1.7.5
patsy: None
dateutil: 2.6.1
pytz: 2018.5
blosc: None
bottleneck: None
tables: 3.4.4
numexpr: 2.6.5
feather: None
matplotlib: 2.2.2
openpyxl: 2.4.9
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 0.9.6
lxml: 4.2.1
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.2.8
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
