gh-85984: Utilize new "winsize" functions from termios in pty tests. (#101831)

Utilize new functions termios.tcgetwinsize() and termios.tcsetwinsize in test_pty.py.

Signed-off-by: Soumendra Ganguly <soumendraganguly@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>