[3.13] gh-127060: Disable traceback colors in IDLE (GH-128028) (#128052)

Set TERM environment variable to "dumb" to disable traceback colors
in IDLE, since IDLE doesn't understand ANSI escape sequences.

(cherry picked from commit 559b0e7013f9cbf42a12fe9c86048d5cbb2f6f22)

Co-authored-by: Victor Stinner <vstinner@python.org>