At CCP We have started using the Py_AddPendingCall() mechanism to 
signal python about a completed IO operation.
However, we noticed that the existing mechanism was hoplelessly un-
thread safe.  This is bad, since on Windows at least, it is very 
convenient to have such callbacks happen on an arbitrary thread from 
the system's thread pool.
I submit a thread-safe implementation instead to be used if WITH_THREAD 
is defined.
This allows Py_AddPendingCall() to be called from any thread, from any 
context, even from a PendingCall callback itself.