[3.13] gh-133256: Add _Py_NONSTRING macro (#133257) (#135135)

gh-133256: Add _Py_NONSTRING macro (#133257)

Fix GCC 15 compiler warnings such as:

    In file included from Python/pylifecycle.c:26:
    Include/internal/pycore_runtime.h:47:26: warning:
    initializer-string for array of 'char' truncates NUL terminator
    but destination lacks 'nonstring' attribute (9 chars into 8
    available) [-Wunterminated-string-initialization]
       47 | #define _Py_Debug_Cookie "xdebugpy"
          |                          ^~~~~~~~~~

(cherry picked from commit e26bafd107aa86a4bdd6051848640f36a56d0efb)