[3.13] gh-116946: fully implement GC protocol for `_curses_panel.panel` (GH-138333) (#138428)

[3.14] gh-116946: fully implement GC protocol for `_curses_panel.panel` (GH-138333)

This commit fixes possible reference loops via `panel.set_userptr`
by implementing `tp_clear` and `tp_traverse` for panel objects.
(cherry picked from commit 572df47840d910b9fc9cd951074232ae89442be1)