gh-145094: fix discards const qualifier from pointer

Since glibc-2.43 and ISO C23, the functions bsearch, memchr, strchr,
strpbrk, strrchr, strstr, wcschr, wcspbrk, wcsrchr, wcsstr and wmemchr
that return pointers into their input arrays now have definitions as
macros that return a pointer to a const-qualified type when the input
argument is a pointer to a const-qualified type.

candidate * pointer returns are only being used for comparisons so declare
them as const, which matches the input variables.

Author: heitbaum

Objects/stringlib/fastsearch.h

@@ -69,8 +69,8 @@ STRINGLIB(find_char)(const STRINGLIB_CHAR* s, Py_ssize_t n, STRINGLIB_CHAR ch)
            and UCS4 representations. */
         if (needle != 0) {
             do {
-                void *candidate = memchr(p, needle,
-                                         (e - p) * sizeof(STRINGLIB_CHAR));
+                const void *candidate = memchr(p, needle,
+                                               (e - p) * sizeof(STRINGLIB_CHAR));
                 if (candidate == NULL)
                     return -1;
                 s1 = p;