For some reason the test_missing_tnull test just out of nowhere started erroring during teardown on Python 3 *only*.  I didn't change anything related to that (just added a new test) and this started.  As far as I can tell there is nothing actually preventing full garbage collection or all files being closed.  I think I've just hit some corner case that puts the garbage collector in a state where it doesn't *quite* close the mmap in time.  Let's just give the teardown a few attempts to remove the files and leave it at that.

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2144 ed100bfc-0583-0410-97f2-c26b58777a21