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Any compiler where sizeof(char) != 1 is *deeply* broken.  In
C, a byte isn't always 8 bits (if it uses bits at all!). 
It's possible for a char to take (for instance) 32 bits, but
sizeof(char) will STILL return 1 in such a case.  A mention
of this in the wild is here:
http://lkml.org/lkml/1998/1/22/4
If you find a compiler that's broken, I'd love to hear about
it. :)

# error Too many signals to fit on an unsigned char!
Should be "in", not "on" :)

A comment in signal_handler() about ignoring the return
value of write() may be good.

initsignal() should avoid not replace
Py_signal_pipe/Py_signal_pipe_w if called a second time
(which is possible, right?).  If so, it should probably not
set them until after setting non-blocking mode.

check_signals() should not call
PyEval_CallObject(Handlers[signum].func, ...) if func is
NULL, which may happen after finisignal() clears it.