#4617: Previously it was illegal to delete a name from the local

namespace if it occurs as a free variable in a nested block.  This limitation
of the compiler has been lifted, and a new opcode introduced (DELETE_DEREF).

This sample was valid in 2.6, but fails to compile in 3.x without this change::

   >>> def f():
   ...     def print_error():
   ...        print(e)
   ...     try:
   ...        something
   ...     except Exception as e:
   ...        print_error()
   ...        # implicit "del e" here


This sample has always been invalid in Python, and now works::

   >>> def outer(x):
   ...     def inner():
   ...        return x
   ...     inner()
   ...     del x

There is no need to bump the PYC magic number: the new opcode is used
for code that did not compile before.

Include/opcode.h

@@ -123,6 +123,7 @@ extern "C" {
 #define LOAD_CLOSURE    135     /* Load free variable from closure */
 #define LOAD_DEREF      136     /* Load and dereference from closure cell */ 
 #define STORE_DEREF     137     /* Store into cell */ 
+#define DELETE_DEREF    138     /* Delete closure cell */ 
 
 /* The next 3 opcodes must be contiguous and satisfy
    (CALL_FUNCTION_VAR - CALL_FUNCTION) & 3 == 1  */