Work around a Solaris peculiarity that caused test_pty to sometimes fail: a

tty opened by os.openpty() isn't always a tty according to os.isatty(), when
it's tested inside the process that opened it. Doesn't affect actual
functionality, as using a tty this way is rarely, if ever, useful. Ignoring
the failure allows the test for actual functionality to continue.

Will backport to 2.4-maint.