[3.14] gh-71880: Allow editing the last cell in curses.textpad.Textbox (GH-152363...

[3.14] gh-71880: Allow editing the last cell in curses.textpad.Textbox (GH-152363) (GH-152365)

Textbox.edit() ignored typing in the lower-right cell of the window.  It is
now written with insch(), which fills the cell without moving the cursor out
of the window (addch() there raises an error and scrolls a scrollable window).
(cherry picked from commit 11b394381f30815a8ad0123afb0a55e7a0369f79)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>