gh-85984: Remove legacy Lib/pty.py code. (#92365)

Refactored the implementation of pty.fork to use os.login_tty.

A DeprecationWarning is now raised by pty.master_open() and pty.slave_open(). They were
undocumented and deprecated long long ago in the docstring in favor of pty.openpty.

Signed-off-by: Soumendra Ganguly <soumendraganguly@gmail.com>
Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>