gh-133256: Add _Py_NONSTRING macro (#133257)

Fix GCC 15 compiler warnings such as:

    Modules/fcntlmodule.c:27:36: warning: initializer-string for
    array of 'char' truncates NUL terminator but destination lacks
    'nonstring' attribute (9 chars into 8 available)
    [-Wunterminated-string-initialization]
    static const char guard[GUARDSZ] = "\x00\xfa\x69\xc4\x67\xa3\x6c\x58";