Changed bytes object ob_sval cast to unsigned char * to prevent a RuntimeError with "Invalid cast"

Cython/Debugger/libcython.py

@@ -578,13 +578,16 @@ class CyCy(CythonCommand):
         cy next
         cy run
         cy cont
+        cy finish
         cy up
         cy down
+        cy select
         cy bt / cy backtrace
-        cy print
         cy list
+        cy print
         cy locals
         cy globals
+        cy exec
     """
     
     name = 'cy'

Cython/Debugger/libpython.py

@@ -48,6 +48,7 @@ from __future__ import with_statement
 import os
 import re
 import sys
+import struct
 import locale
 import atexit
 import warnings
@@ -1026,8 +1027,8 @@ class PyBytesObjectPtr(PyObjectPtr):
     def __str__(self):
         field_ob_size = self.field('ob_size')
         field_ob_sval = self.field('ob_sval')
-        char_ptr = field_ob_sval.address.cast(_type_unsigned_char_ptr)
-        return ''.join([chr(char_ptr[i]) for i in safe_range(field_ob_size)])
+        return ''.join(struct.pack('b', field_ob_sval[i]) 
+                           for i in safe_range(field_ob_size))
 
     def proxyval(self, visited):
         return str(self)