Here is a patch that moves the source files of the _io module (and
closely related modules) into subdirectory of Modules. The new file
hierarchy proposed is the following:

  Modules/
    _io/
      _iomodule.h
      _iomodule.c
      iobase.c
      fileio.c
      bufferedio.c
      textio.c
      bytesio.c
      stringio.c

I believe that moving the io module into its own subdirectory will make
clear whether the different I/O implementations share common bugs.
Currently, it is quite easy to miss a module when doing a bug fix.

The patch was not tested on Windows. I tried my best to configure by
hand PCbuild and PC/{VC, VS7.1, VS8.0}. However, I would be surprised if
my changes were flawless.