TST: fix AttributeError: 'module' object has no attribute 'nl_langinfo' on Windows

[cgohlke]

Fixes one test error:

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ERROR: Tests that DateFormatter matches datetime.strftime,
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Traceback (most recent call last):
  File "X:\Python34\lib\site-packages\nose\case.py", line 198, in runTest
    self.test(*self.arg)
  File "X:\Python34\lib\site-packages\matplotlib\tests\test_dates.py", line 207, in test_date_formatter_strftime
    test_strftime_fields(datetime.datetime(year, 1, 1))
  File "X:\Python34\lib\site-packages\matplotlib\tests\test_dates.py", line 198, in test_strftime_fields
    locale_d_fmt = locale.nl_langinfo(locale.D_FMT)
AttributeError: 'module' object has no attribute 'nl_langinfo'

[tacaswell]

backported as 2ac4536