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I've looked over the patch, although I haven't tested it.  I
have the following suggestions:

* Needs documentation explaining the signal weirdness (may
drop signals, may delay indefinitely, new handlers may get
signals intended for old, etc)
* Needs to be explicit that users must only poll/select to
check for readability of the pipe, NOT read from it
* The comment for is_tripped refers to sigcheck(), which
doesn't exist
* I think we should be more paranoid about the range of
possible signals.  NSIG does not appear to be defined by
SUSv2 (no clue about Posix).  We should size the Handlers
array to UCHAR_MAX and set any signals outside the range of
0..UCHAR_MAX to either 0 (null signal) or UCHAR_MAX.  I'm
not sure we should ever use NSIG.
* In signal_hander() sizeof(signum_c) is inherently 1. ;)
* The set_nonblock macro doesn't check for errors from
fcntl().  I'm not sure it's worth having a macro for that
anyway.
* Needs some documentation of the assumptions about
read()/write() being memory barriers.
* In check_signals() sizeof(signum) is inherently 1.
* There's a blank line with tabs near the end of
check_signals() ;)
* PyErr_SetInterrupt() should use a compile-time check for
SIGINT being within 0..UCHAR_MAX, assuming NSIG is ripped
out entierly.
* PyErr_SetInterrupt() needs to set is_tripped after the
call to write(), not before.
* PyOS_InterruptOccurred() should probably still check that
it's called from the main thread.