If an application embeds Python while stderr is not
valid, warnings can cause unexpected exceptions in the
application.  This is particularly a problem when it
happens *while* the application is attempting to create
a valid stderr :)

Pythonwin recently started failing to start with the
following traceback:

  File "f:\src\python-cvs\lib\regsub.py", line 15, in ?
    DeprecationWarning)
  File "f:\src\python-cvs\lib\warnings.py", line 45, in
warn
    warn_explicit(message, category, filename, lineno,
module, registry)
  File "f:\src\python-cvs\lib\warnings.py", line 104,
in warn_explicit
    showwarning(message, category, filename, lineno)
  File "f:\src\python-cvs\lib\warnings.py", line 110,
in showwarning
    file.write(formatwarning(message, category,
filename, lineno))
exceptions.IOError: (9, 'Bad file descriptor')

I propose that the warnings module ignore IOError
exceptions when writing the warning.  Attaching
proposed fix.

Assigning to Martin for feedback - assign back for me
to check in.