siginterrupt is not avalible on windows so make it optional

Author: Vizonex

uvloop/includes/stdlib.pxi

@@ -150,7 +150,7 @@ cdef int signal_SIGABRT = signal.SIGABRT
 cdef int signal_SIGINT = signal.SIGINT
 cdef int signal_NSIG = signal.NSIG
 cdef signal_signal = signal.signal
-cdef signal_siginterrupt = signal.siginterrupt
+cdef signal_siginterrupt = getattr(signal, "siginterrupt", None)
 cdef signal_set_wakeup_fd = signal.set_wakeup_fd
 cdef signal_default_int_handler = signal.default_int_handler
 cdef signal_SIG_DFL = signal.SIG_DFL