- The patch modifies the _io module but not _pyio, why?
(try   f=_pyio.open("XX","w+b")   at the beginning of the script above)

- One test was *removed*, but nothing was added to test this new feature.
This is the most likely way to lose it in future versions!
An idea for the test is to do like the MockRawIO class in test_io.py, which "implements" a file but fakes all system calls, and can be wrapped in a BufferedReader.

- The feature seems to be not applicable to text files, this should be tested.