pep-550: Fix PEP 521 example. (#337)

pep-0550.rst

@@ -942,30 +942,32 @@ The following code::
 
 would become this::
 
+    local = threading.local()
+
     class Context:
 
         def __enter__(self):
-            self.old_x = get_execution_context_item('x')
-            set_execution_context_item('x', 'something')
+            self.old_x = getattr(local, 'x', None)
+            local.x = 'something'
 
         def __suspend__(self):
-            set_execution_context_item('x', self.old_x)
+            local.x = self.old_x
 
         def __resume__(self):
-            set_execution_context_item('x', 'something')
+            local.x = 'something'
 
         def __exit__(self, *err):
-            set_execution_context_item('x', self.old_x)
+            local.x = self.old_x
 
 Besides complicating the protocol, the implementation will likely
 negatively impact performance of coroutines, generators, and any code
 that uses context managers, and will notably complicate the
-interpreter implementation.  It also does not solve the leaking state
-problem for greenlet/gevent.
+interpreter implementation.
 
 :pep:`521` also does not provide any mechanism to propagate state
 in a local context, like storing a request object in an HTTP request
-handler to have better logging.
+handler to have better logging.  Nor does it solve the leaking state
+problem for greenlet/gevent.
 
 
 Can Execution Context be implemented outside of CPython?