Oops.
Victor, my mouse got stuck and I mistakenly removed your pending_signals-2 patch. I'm really sorry about this, could you re-post it?

To try to make up for this, a small comment:
In signal_sigwait, at the end of the function,  you do this:

    /* call the signal handler (if any) */
    if (PyErr_CheckSignals())
        return NULL;

I'm not sure I get this: sigwait is used to handle signals synchronously, and in the POSIX semantic, it's incompatible with signal handlers:

"""
sigwait suspends the calling thread until one of the signals in set is delivered to the calling thread. It then stores the number of the signal received in the location pointed to by sig and returns. The signals in set must be blocked and not ignored on entrance to sigwait. If the delivered signal has a signal handler function attached, that function is *not* called.
"""

and
"""
The effect of sigwait() on the signal actions for the signals in set is unspecified.
"""

So, if anything, you shouldn't check for a pending signal.