> we should make sure that it's not possible to load an extension
> compiled with 3.1 in 3.2 to prevent segfaults and buffer overruns.

This is the case with this patch: today all these functions
(_PyUnicode_IsAlpha, _PyUnicode_ToLowercase) are actually #defines to
_PyUnicodeUCS2_* or _PyUnicodeUCS4_*.
The patch removes the #defines: 3.1 modules that call
_PyUnicodeUCS4_IsAlpha wouldn't load into a 3.2 interpreter.

> The change affects the Unicode type database which is implemented
> in unicodectype.c, not the Unicode database, which already uses UCS4.

Are you referring to the _PyUnicode_TypeRecord structure?
The first three fields only contains values up to 65535, so they could
use "unsigned short" even for UCS4 builds.
All the other uses are precisely changed by the patch...