docs: def setblocking(fd, blocking):
      """Set the O_NONBLOCK flag for a file descriptor. Availability: Unix."""
      if not fcntl:
          warnings.warn('setblocking() not supported on Windows')
      flags = fcntl.fcntl(fd, fcntl.F_GETFL)
      if blocking:
          flags |= os.O_NONBLOCK
      else:
          flags &= ~os.O_NONBLOCK
      fcntl.fcntl(fd, fcntl.F_SETFL, flags)