Add missing warn_on_full_buffer argument to signal.set_wakeup_fd() (#4377)

According to
https://docs.python.org/3/library/signal.html#signal.set_wakeup_fd, this
was added in Python 3.7.

stdlib/3/signal.pyi

@@ -177,7 +177,11 @@ if sys.platform != "win32":
     def pthread_kill(__thread_id: int, __signalnum: int) -> None: ...
     def pthread_sigmask(__how: int, __mask: Iterable[int]) -> Set[_SIGNUM]: ...
 
-def set_wakeup_fd(fd: int) -> int: ...
+if sys.version_info >= (3, 7):
+    def set_wakeup_fd(fd: int, *, warn_on_full_buffer: bool = ...) -> int: ...
+
+else:
+    def set_wakeup_fd(fd: int) -> int: ...
 
 if sys.platform != "win32":
     def setitimer(__which: int, __seconds: float, __interval: float = ...) -> Tuple[float, float]: ...