gh-89653: PEP 670: Convert PyUnicode_KIND() macro to function (#92705)

In the limited C API version 3.12, PyUnicode_KIND() is now
implemented as a static inline function. Keep the macro for the
regular C API and for the limited C API version 3.11 and older to
prevent introducing new compiler warnings.

Update _decimal.c and stringlib/eq.h for PyUnicode_KIND().