Fix compiler warning in _ctypes_test on strchr()

[vstinner]

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*".

[vstinner]

See for example the glibc commit which introduced C23 support in string/string.h.

The code is now:

extern char *strchr (const char *__s, int __c)
     __THROW __attribute_pure__ __nonnull ((1));
# if __GLIBC_USE (ISOC23) && defined __glibc_const_generic && !defined _LIBC
#  define strchr(S, C)                                         \
  __glibc_const_generic (S, const char *, strchr (S, C))
# endif