bpo-26228: Fix pty EOF handling (GH-12049)

On non-Linux POSIX platforms, like FreeBSD or macOS,
the FD used to read a forked PTY may signal its exit not
by raising an error but by sending empty data to the read
syscall. This case wasn't handled, leading to hanging
`pty.spawn` calls.

Co-authored-by: Reilly Tucker Siemens <reilly@tuckersiemens.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>