gh-106693: Explicitly mark ob_sval as unsigned char to avoid UB

Author: pablogsal

Include/cpython/bytesobject.h

@@ -5,7 +5,7 @@
 typedef struct {
     PyObject_VAR_HEAD
     Py_DEPRECATED(3.11) Py_hash_t ob_shash;
-    char ob_sval[1];
+    unsigned char ob_sval[1];
 
     /* Invariants:
      *     ob_sval contains space for 'ob_size+1' elements.