fix mac os x non blocking error

[zer0pwned]

For some reason the setnonblocking function failed on both my Intel and M1 based Macbook Pro. The fcntl(fd, F_SETFL, mode) complains about ENOTTY but for a fact that the fd is already non-block mode and simply works for pty communication without any issue.

[zer0pwned]

I can see that the test run preformed over here https://github.com/cheatfate/asynctools/runs/2940556595 also suffers this issue.