gh-85984: Add POSIX pseudo-terminal functions.

[8vasu]

This follows #101831. This is one in a series of PRs aimed at cleaning-up, fixing bugs in, introducing new features in, and updating the code in "Lib/pty.py".

This PR answers the following question: os.forkpty() and pty.fork() return a pair pid, fd, where fd is a file descriptor of the master end of a pseudo-terminal pair; if at a later stage one needs to make some modifications to the slave end (such as setting termios attributes), then how does one obtain access to it without reimplementing ptsname() in Python or loading it from a shared library like someone is doing here: https://stackoverflow.com/questions/52338062/calling-libc-select-from-python-from-pty-master-side?

This is a dependency of #101833, which only needs os.ptsname(). However, this PR also adds os.posix_openpt(), os.grantpt(), and os.unlockpt() since all of these POSIX functions are "companions" of each other.

Signed-off-by: Soumendra Ganguly soumendraganguly@gmail.com

• Issue: Lib/pty.py major revision #85984