On Windows there are tiny delay between call to os.unlink and real file 
removing. Periodically it leads to unittest crashes in cases like this:

test.support.unlink(filename)
f = open(filename, 'wb')

Proposed solution: wait in support.unlink for end of deletion asking 
os.stat for removed file (only if os.name == 'nt', of course).

Also test.test_linecache:LineCacheTests.test_checkcache should be fixed 
- this one miss to close last opened file and Windows cannot remove it.

Both patches for trunk and py3k is attached.