gh-149707: Fix compiler warning in _ctypes_test on strchr() (GH-149791)

Change my_strchr() return type to "const char*" (add "const").

Fix the compiler warning:

Modules/_ctypes/_ctypes_test.c: In function 'my_strchr':
Modules/_ctypes/_ctypes_test.c:451:12: warning: return discards
'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  451 |     return strchr(s, c);
      |            ^~~~~~

When using C23, strchr(text, ch) return type is "const char*" if text
type is "const char*".
(cherry picked from commit 5465b69)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: vstinner

Modules/_ctypes/_ctypes_test.c

@@ -405,7 +405,7 @@ EXPORT(char *)my_strtok(char *token, const char *delim)
     return strtok(token, delim);
 }
 
-EXPORT(char *)my_strchr(const char *s, int c)
+EXPORT(const char *) my_strchr(const char *s, int c)
 {
     return strchr(s, c);
 }