[3.14] gh-134466: Don't run when termios is inaccessible (GH-138911) (GH-139029)

Without the ability to set required capabilities, the REPL cannot
function properly (syntax highlighting and multiline editing can't
work).
We refuse to work in this degraded state.

(cherry picked from commit 2fc7004d5437e7bb0a1f5b962be441ef0ee7434b)

Co-authored-by: Łukasz Langa <lukasz@langa.pl>