Issue #26765: Ensure that bytes- and unicode-specific stringlib files are used

with correct type.

Objects/stringlib/codecs.h

 /* stringlib: codec implementations */
 
-#if STRINGLIB_IS_UNICODE
+#if !STRINGLIB_IS_UNICODE
+# error "codecs.h is specific to Unicode"
+#endif
 
 /* Mask to quickly check whether a C 'long' contains a
    non-ASCII, UTF8-encoded char. */
@@ -823,5 +825,3 @@ STRINGLIB(utf32_encode)(const STRINGLIB_CHAR *in,
 #undef SWAB4
 
 #endif
-
-#endif /* STRINGLIB_IS_UNICODE */

Objects/stringlib/ctype.h

-/* NOTE: this API is -ONLY- for use with single byte character strings. */
-/* Do not use it with Unicode. */
+#if STRINGLIB_IS_UNICODE
+# error "ctype.h only compatible with byte-wise strings"
+#endif
 
 #include "bytes_methods.h"
 

Objects/stringlib/find_max_char.h

 /* Finding the optimal width of unicode characters in a buffer */
 
-#if STRINGLIB_IS_UNICODE
+#if !STRINGLIB_IS_UNICODE
+# error "find_max_char.h is specific to Unicode"
+#endif
 
 /* Mask to quickly check whether a C 'long' contains a
    non-ASCII, UTF8-encoded char. */
@@ -129,5 +131,4 @@ STRINGLIB(find_max_char)(const STRINGLIB_CHAR *begin, const STRINGLIB_CHAR *end)
 #undef MAX_CHAR_UCS4
 
 #endif /* STRINGLIB_SIZEOF_CHAR == 1 */
-#endif /* STRINGLIB_IS_UNICODE */
 

Objects/stringlib/join.h

 /* stringlib: bytes joining implementation */
 
-#if STRINGLIB_SIZEOF_CHAR != 1
+#if STRINGLIB_IS_UNICODE
 #error join.h only compatible with byte-wise strings
 #endif
 

Objects/stringlib/localeutil.h

@@ -2,8 +2,8 @@
 
 #include <locale.h>
 
-#ifndef STRINGLIB_IS_UNICODE
-#   error "localeutil is specific to Unicode"
+#if !STRINGLIB_IS_UNICODE
+#   error "localeutil.h is specific to Unicode"
 #endif
 
 typedef struct {

Objects/stringlib/transmogrify.h

-/* NOTE: this API is -ONLY- for use with single byte character strings. */
-/* Do not use it with Unicode. */
+#if STRINGLIB_IS_UNICODE
+# error "transmogrify.h only compatible with byte-wise strings"
+#endif
 
 /* the more complicated methods.  parts of these should be pulled out into the
    shared code in bytes_methods.c to cut down on duplicate code bloat.  */