[3.15] gh-134300: Remove idlelib from the path of the IDLE user process (GH-152739...

[3.15] gh-134300: Remove idlelib from the path of the IDLE user process (GH-152739) (#152807)

gh-134300: Remove idlelib from the path of the IDLE user process (GH-152739)

The idlelib directory ends up on sys.path when idle.py is run as a script,
and it was passed to the user process, where it let user code import
idlelib submodules as top-level modules, such as "import help".
(cherry picked from commit 3f5491a09223f7371b4b4c92225e8dfdf1c989b7)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>