Fix WindowsError: [Error 32] The process cannot access the file

Fixes several errors in `test_backend_pdf.test_multipage_keep_empty` on Windows, where files can not be deleted while being open. For example:
```
======================================================================
ERROR: matplotlib.tests.test_backend_pdf.test_multipage_keep_empty
----------------------------------------------------------------------
Traceback (most recent call last):
  File "X:\Python27-x64\lib\site-packages\nose\case.py", line 197, in runTest
    self.test(*self.arg)
  File "X:\Python27-x64\lib\site-packages\matplotlib\testing\decorators.py", line 110, in wrapped_function
    func(*args, **kwargs)
  File "X:\Python27-x64\lib\site-packages\matplotlib\tests\test_backend_pdf.py", line 67, in test_multipage_keep_empty
    os.remove(filename)
WindowsError: [Error 32] The process cannot access the file because it is being used by another process: 'c:\\users\\gohlke\\appdata\\local\\temp\\tmpc9vzti'
```