gh-149707: Fix compiler warning in _ctypes_test on strchr() (#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*".