#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.

Lib/test/test_exceptions.py

@@ -526,6 +526,17 @@ class ExceptionTests(unittest.TestCase):
         obj = wr()
         self.assertTrue(obj is None, "%s" % obj)
 
+    def test_exception_target_in_nested_scope(self):
+        # issue 4617: This used to raise a SyntaxError
+        # "can not delete variable 'e' referenced in nested scope"
+        def print_error():
+            e
+        try:
+            something
+        except Exception as e:
+            print_error()
+            # implicit "del e" here
+
     def test_generator_leaking(self):
         # Test that generator exception state doesn't leak into the calling
         # frame