I just hit this as well when rerunning the 2.5 tests before checking
something else in. The test itself appears to be fine, but the call to
f.close() outside the try/except block attempting to flush the file to
disk and raising an IOError.

Didn't something like this get fixed recently? Did the new IO module in
py3k have a similar problem?

(assigning to Neal to make a call on the importance for 2.5.2)