#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.

Python/opcode_targets.h

@@ -137,7 +137,7 @@ static void *opcode_targets[256] = {
     &&TARGET_LOAD_CLOSURE,
     &&TARGET_LOAD_DEREF,
     &&TARGET_STORE_DEREF,
-    &&_unknown_opcode,
+    &&TARGET_DELETE_DEREF,
     &&_unknown_opcode,
     &&TARGET_CALL_FUNCTION_VAR,
     &&TARGET_CALL_FUNCTION_KW,