[3.15] gh-153862: Fix curses window.inch() for non-ASCII characters on a wide build...

[3.15] gh-153862: Fix curses window.inch() for non-ASCII characters on a wide build (GH-153863) (GH-154116)

On a wide build, winch() returns the low 8 bits of the character's code
point with no locale conversion, so inch()/mvinch() disagreed with instr()
for a non-ASCII character of an 8-bit locale ('€' under ISO-8859-15 gave
0xAC instead of 0xA4).  Re-encode the cell to its locale byte via wctob(),
as ncurses does for getbkgd().

(cherry picked from commit a83aaa91490d5885949eb0780a37b15963b6d0f3)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>